checksums: add SHA-512 and a Checksums-* field parser

The checksum model only carried md5/sha1/sha256 while deb-buildinfo(5)
defines Checksums-Sha512, and there was no way to parse a Checksums-*
field body back into entries. Add ChecksumKind::Sha512 (computed
alongside the others), a field parser validating the
'<hex> <size> <name>' grammar, and an only-if-populated
Checksums-Sha512 emission in .buildinfo — deliberately dormant in the
dpkg-parity flows, which never emit it, and .dsc/.changes untouched.

src/debian/mod.rs

@@ -23,7 +23,7 @@ pub use changelog::{
     ChangelogEntry, parse_changelog_entry, parse_changelog_entry_from_str,
     parse_previous_version_from_str,
 };
-pub use checksums::{Entry as ChecksumEntry, FileChecksums};
+pub use checksums::{ChecksumKind, Entry as ChecksumEntry, FileChecksums};
 pub use control::{
     ControlInfo, Paragraph, parse_paragraphs, strip_clearsigned_armour, write_paragraph,
 };