* build.zig: the result of b.option() can be assigned directly in many
   cases thanks to the return type being an optional
 * std.Build: make the build system aware of the
   std.Build.Step.Compile.BuildId type when used as an option.
   - remove extraneous newlines in error logs
 * simplify caching logic
 * simplify hexstring parsing tests and use a doc test
 * simplify hashing logic. don't use an optional when the `none` tag
   already provides this meaning.
 * CLI: fix incorrect linker arg parsing