Change API of parse_env functions

I think this is an overall improvement. A few things done here:
- They are available as `::env::parse` instead of `::env_utils`. The
  `env` module gets functions of its own soon enough.
- Renamed functions to be shorter: `list_by_sep`, `list_by_comma`, ...
- The docs were adjusted. One example is enough. And the functions with
  a fixed separator don't need the full docs again. That way we can
  avoid the `paste` dependency.
- Functions now always return `Result<C, _>`. While the previous version
  was slightly more flexible, I don't think anyone would benefit from
  that flexibility (as `parse_env` requires `Result<_, _>` anyway) and
  this way it's a bit clearer what the function does, especially for
  those who don't know the nifty `FromIterator for Result` impl.
- The example was adjusted accordingly. I also changed the names to
  obviously dummy names as I didn't know the existing names and don't
  want to spend time investigating whether I want their names in my
  code base :D

examples/parse_env.rs

@@ -1,39 +1,24 @@
 #![allow(dead_code)]
 
-use confique::{
-    env_utils::{
-        to_collection_by_char_separator, to_collection_by_comma, to_collection_by_semicolon,
-    },
-    Config,
-};
+use confique::Config;
 use std::{collections::HashSet, num::NonZeroU64, path::PathBuf, str::FromStr, convert::Infallible};
 
+
 #[derive(Debug, Config)]
 struct Conf {
-    #[config(
-        env = "PATHS",
-        parse_env = to_collection_by_comma
-    )]
+    #[config(env = "PATHS", parse_env = confique::env::parse::list_by_colon)]
     paths: HashSet<PathBuf>,
-    #[config(
-         env = "PORTS",
-         parse_env = to_collection_by_semicolon
-    )]
+
+    #[config(env = "PORTS", parse_env = confique::env::parse::list_by_comma)]
     ports: Vec<u16>,
-    #[config(
-         env = "NAMES",
-         parse_env = to_collection_by_char_separator::<'|', _, _>
-    )]
+
+    #[config(env = "NAMES", parse_env = confique::env::parse::list_by_sep::<'|', _, _>)]
     names: Vec<String>,
-    #[config(
-         env = "TIMEOUT",
-         parse_env = NonZeroU64::from_str,
-    )]
+
+    #[config(env = "TIMEOUT", parse_env = NonZeroU64::from_str)]
     timeout_seconds: NonZeroU64,
-    #[config(
-         env = "FORMATS",
-         parse_env = parse_formats,
-    )]
+
+    #[config(env = "FORMATS", parse_env = parse_formats)]
     formats: Vec<Format>,
 }
 
@@ -45,6 +30,7 @@ enum Format {
     Yaml,
 }
 
+/// Example custom parser.
 fn parse_formats(input: &str) -> Result<Vec<Format>, Infallible> {
     let mut result = Vec::new();
 
@@ -66,11 +52,8 @@ fn parse_formats(input: &str) -> Result<Vec<Format>, Infallible> {
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     std::env::set_var("PATHS", "/bin/ls,/usr/local/bin,/usr/bin/ls");
-    std::env::set_var("PORTS", "8080;8888;8000");
-    std::env::set_var(
-        "NAMES",
-        "Mervinc Harmon|Alfreda Valenzuela|Arlen Cabrera|Damon Rice|Willie Schwartz",
-    );
+    std::env::set_var("PORTS", "8080,8888,8000");
+    std::env::set_var("NAMES", "Alex|Peter|Mary");
     std::env::set_var("TIMEOUT", "100");
     std::env::set_var("FORMATS", "json5,yaml;.env");