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/array-queue/RUSTSEC-2020-0047.md

@@ -1,17 +1,18 @@
+```toml
 [advisory]
 id = "RUSTSEC-2020-0047"
 package = "array-queue"
 date = "2020-09-26"
-title = "array_queue pop_back() may cause a use-after-free"
+keywords = ["memory-corruption", "uninitialized-memory", "use-after-free"]
 url = "https://github.com/raviqqe/array-queue/issues/2"
-description = """
+
+[versions]
+patched = []
+unaffected = ["< 0.3.0"]
+```
+
+# array_queue pop_back() may cause a use-after-free
+
 array_queue implements a circular queue that wraps around an array. However, it
 fails to properly index into the array in the `pop_back` function allowing the
 reading of previously dropped or uninitialized memory.
-"""
-keywords = ["memory-corruption", "uninitialized-memory", "use-after-free"]
-
-[versions]
-
-patched = []
-unaffected = ["< 0.3.0"]