Translate database into V3 advisory format (#420)

As proposed in #240 and tracked in #414, this PR translates all
advisories into the new "V3" advisory format, which is based on Markdown
with leading TOML front matter.

This format makes it easier to see rendered Markdown syntax
descriptions, whether rendered by an IDE or GitHub. This should help
with both crafting advisories initially as well as review, and ideally
encourages more lengthy descriptions.

Support for this format shipped in `cargo-audit` v0.12.0 on
May 6th, 2020.

crates/actix-codec/RUSTSEC-2020-0049.md

@@ -1,17 +1,18 @@
+```toml
 [advisory]
 id = "RUSTSEC-2020-0049"
 package = "actix-codec"
-date = "2020-01-30"
-title = "Use-after-free in Framed due to lack of pinning"
-url = "https://github.com/actix/actix-net/issues/91"
 categories = ["memory-corruption"]
+date = "2020-01-30"
+url = "https://github.com/actix/actix-net/issues/91"
+
+[versions]
+patched = [">= 0.3.0-beta.1"]
+```
+
+# Use-after-free in Framed due to lack of pinning
 
-description = """
 Affected versions of this crate did not require the buffer wrapped in `Framed` to be pinned,
 but treated it as if it had a fixed location in memory. This may result in a use-after-free.
  
 The flaw was corrected by making the affected functions accept `Pin<&mut Self>` instead of `&mut self`.
-"""
-
-[versions]
-patched = [">= 0.3.0-beta.1"]