check that fn decls in impls/traits match

[nikomatsakis]

Very basic check that function declarations in impls vs traits match. Checks:

• impl and trait version of fn have the same generic parameters (this is too strict, compared to rustc, because of late-bound lifetimes)
• parameter types on the trait are a subtype of parameter types on the impl
• return type on the impl is a subtype of return type on the trait

I can't remember precisely how this check compares to rustc! This is the most general version, but I think rustc requires equality perhaps in some places. We should figure that out.

Fixes #69