Add signature search

[fredemmott]

For example, searching for:

• (string): string could return Str\uppercase
• (vec<Ta>): vec<Tb> could return Vec\map
• (vec<_>): vec<_> should also return Vec\map
• (dict<_, _>): vec<_> should also return Vec\map
• (vec<_>): int should return C\count

This should have a basic understanding of generics and variance (e.g. if I search for vec<T> as a parameter type that should include Container<T>); _ could be implemented as a universal base type instead of a special-cased subtype

[fredemmott]

https://www.haskell.org/hoogle/ is comparable

[fredemmott]

This would likely involve changes to:

• src/api-gen and src/hh-apidocs to include signatures in metadata
• src/search to implement the actual search