Allow mocking non-'static structs

[asomers]

Constructor methods are not yet supported.

Fixes #4

The gory details:

• Generics of mock structs should not include lifetimes
• deselfify before split lifetimes
• Include rlifetimes in some impl blocks
• Include some missing <> in an intermediate Generics object
• Fix deselfify with lifetimes
• Handle apparently unused lifetimes in split_lifetimes
• Fix determination of whether a method is generic in method_types. There was
  some leftover logic from back when all methods of generic structs were
  treated as generic methods.
• Remove lifetimes from Context objects