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/string-interner/RUSTSEC-2019-0023.md

@@ -1,11 +1,18 @@
+```toml
 [advisory]
 id = "RUSTSEC-2019-0023"
 package = "string-interner"
+aliases = ["CVE-2019-16882"]
 date = "2019-08-24"
-title = "Cloned interners may read already dropped strings"
-url = "https://github.com/Robbepop/string-interner/issues/9"
 keywords = ["use after free"]
-description = """
+url = "https://github.com/Robbepop/string-interner/issues/9"
+
+[versions]
+patched = ["^0.6.4", ">= 0.7.1"]
+```
+
+# Cloned interners may read already dropped strings
+
 Affected versions of this crate did not clone contained strings when an interner is cloned.
 Interners have raw pointers to the contained strings, and they keep pointing the strings which the old interner owns, after the interner is cloned.
 If a new cloned interner is alive and the old original interner is dead, the new interner has dangling pointers to the old interner's storage, which is already dropped.
@@ -19,8 +26,3 @@ This patch implements `Clone` manually to the interner type, so that the interna
 
 PR #10 was also backported to the 0.6 release line in
 <https://github.com/Robbepop/string-interner/pull/14> and was released in 0.6.4.
-"""
-aliases = ["CVE-2019-16882"]
-
-[versions]
-patched = ["^0.6.4", ">= 0.7.1"]