While this is less readable, I spent way too long trying to adapt
the bindings generation code to handle glob imports and concluded
it would take refactoring almost the entire import-resolution
logic. While this may be a good refactor to do eventually, its
probably not worth it today.
This prevents aliasing the global secp256k1::Signature name in C
bindings and also makes it a little more explicit that the object
is different from other signature types.
