Simplify schema module further.

[obmarg]

Even with pre-registration, generating and compiling large schemas isn't as fast as it could be. Some of that slow down is generating & subsequently compiling the use_schema output.

Looking at the shape of the output I think there might be room to simplify it:

• The Field trait could probably be dropped in favour of HasField & HasInputField
• HasInputField might not need its second generic parameter.

These changes would probably reduce the output size quite significantly which would provide a fair bit of speed up. Likely a breaking change, but not one that would affect most users.