Remove deprecated tokio-proto and replace with homegrown rpc framework (#199)

# New Crates

- crate rpc contains the core client/server request-response framework, as well as a transport trait.
- crate bincode-transport implements a transport that works almost exactly as tarpc works today (not to say it's wire-compatible).
- crate trace has some foundational types for tracing. This isn't really fleshed out yet, but it's useful for in-process log tracing, at least.

All crates are now at the top level. e.g. tarpc-plugins is now tarpc/plugins rather than tarpc/src/plugins. tarpc itself is now a *very* small code surface, as most functionality has been moved into the other more granular crates.

# New Features
- deadlines: all requests specify a deadline, and a server will stop processing a response when past its deadline.
- client cancellation propagation: when a client drops a request, the client sends a message to the server informing it to cancel its response. This means cancellations can propagate across multiple server hops.
- trace context stuff as mentioned above
- more server configuration for total connection limits, per-connection request limits, etc.

# Removals
- no more shutdown handle.  I left it out for now because of time and not being sure what the right solution is.
- all async now, no blocking stub or server interface. This helps with maintainability, and async/await makes async code much more usable. The service trait is thusly renamed Service, and the client is renamed Client.
- no built-in transport. Tarpc is now transport agnostic (see bincode-transport for transitioning existing uses).
- going along with the previous bullet, no preferred transport means no TLS support at this time. We could make a tls transport or make bincode-transport compatible with TLS.
- a lot of examples were removed because I couldn't keep up with maintaining all of them. Hopefully the ones I kept are still illustrative.
- no more plugins!

# Open Questions

1. Should client.send() return `Future<Response>` or `Future<Future<Response>>`? The former appears more ergonomic but it doesn’t allow concurrent requests with a single client handle. The latter is less ergonomic but yields back control of the client once it’s successfully sent out the request. Should we offer fns for both?
2. Should rpc service! Fns take &mut self or &self or self? The service needs to impl Clone anyway, technically we only need to clone it once per connection, and then leave it up to the user to decide if they want to clone it per RPC. In practice, everyone doing nontrivial stuff will need to clone it per RPC, I think.
3. Do the request/response structs look ok?
4. Is supporting server shutdown/lameduck important?

Fixes #178 #155 #124 #104 #83 #38

plugins/src/lib.rs

@@ -0,0 +1,85 @@
+extern crate proc_macro;
+extern crate proc_macro2;
+extern crate syn;
+extern crate itertools;
+extern crate quote;
+
+use proc_macro::TokenStream;
+
+use itertools::Itertools;
+use quote::ToTokens;
+use syn::{Ident, TraitItemType, TypePath, parse};
+use proc_macro2::Span;
+use std::str::FromStr;
+
+#[proc_macro]
+pub fn snake_to_camel(input: TokenStream) -> TokenStream {
+    let i = input.clone();
+    let mut assoc_type = parse::<TraitItemType>(input).unwrap_or_else(|_| panic!("Could not parse trait item from:\n{}", i));
+
+    let old_ident = convert(&mut assoc_type.ident);
+
+    for mut attr in &mut assoc_type.attrs {
+        if let Some(pair) = attr.path.segments.first() {
+            if pair.value().ident == "doc" {
+                attr.tts = proc_macro2::TokenStream::from_str(&attr.tts.to_string().replace("{}", &old_ident)).unwrap();
+            }
+        }
+    }
+
+    assoc_type.into_token_stream().into()
+}
+
+#[proc_macro]
+pub fn ty_snake_to_camel(input: TokenStream) -> TokenStream {
+    let mut path = parse::<TypePath>(input).unwrap();
+
+    // Only capitalize the final segment
+    convert(&mut path.path
+                     .segments
+                     .last_mut()
+                     .unwrap()
+                     .into_value()
+                     .ident);
+
+    path.into_token_stream().into()
+}
+
+/// Converts an ident in-place to CamelCase and returns the previous ident.
+fn convert(ident: &mut Ident) -> String {
+    let ident_str = ident.to_string();
+    let mut camel_ty = String::new();
+
+    {
+        // Find the first non-underscore and add it capitalized.
+        let mut chars = ident_str.chars();
+
+        // Find the first non-underscore char, uppercase it, and append it.
+        // Guaranteed to succeed because all idents must have at least one non-underscore char.
+        camel_ty.extend(chars.find(|&c| c != '_').unwrap().to_uppercase());
+
+        // When we find an underscore, we remove it and capitalize the next char. To do this,
+        // we need to ensure the next char is not another underscore.
+        let mut chars = chars.coalesce(|c1, c2| {
+            if c1 == '_' && c2 == '_' {
+                Ok(c1)
+            } else {
+                Err((c1, c2))
+            }
+        });
+
+        while let Some(c) = chars.next() {
+            if c != '_' {
+                camel_ty.push(c);
+            } else if let Some(c) = chars.next() {
+                camel_ty.extend(c.to_uppercase());
+            }
+        }
+    }
+
+    // The Fut suffix is hardcoded right now; this macro isn't really meant to be general-purpose.
+    camel_ty.push_str("Fut");
+
+    *ident = Ident::new(&camel_ty, Span::call_site());
+    ident_str
+}