Use diffy to apply patches instead of libgit2

Using diffy allows for applying patches with incorrect line numbers. This is what the standard git-apply command does and also GNU patch. This feature was missing from recent versions of libgit2 (but appeared to be in old ones). See PR #4 for more details.

This is by far the biggest change. Note that libgit2 is still a required dependency.

• Switch parsing from regex to nom
• Improve error messages (hopefully) by using thiserror
• By default, use static linking for dependencies
• Move parsing logic out of apply/regenerate module
  • Hopefully makes more usable as a library
• Use vergen to include git-describe info into clap CLI
  • Implement a --version flag for the CLI

WARNING: This release is effectively broken, because it does not support stable rust. I foolishly published before running ./dist.sh. For this reason, there are no published binaries and it has been yanked from https://crates.io. Prefer v0.2.1, which differs only slightly and has included a fix.