Simplify internal function signature representation

[scrabsha]

Similarly to #4, we need to drop information from the syn-generated AST, so that we avoid a lot incorrectly-detected breaking changes. Ideally we keep only the generic parameters, input type and output type. See at the end of the issue for an example of non-breaking change that is currently flagged as breaking.

Ideally this would be done after #4, but this is not mandatory.

This issue is part of #2.

Before

fn f(
    i: u32,
) {}

After

fn f(
    i: u32
) {}