Change mock!'s syntax for traits

[asomers]

Previously the syntax looked like a trait definition `trait X`.  But
that's ambiguous in the case of generic traits.  It doesn't make clear
the relationship between the trait's generic parameters and the struct's
generic parameters.  And it made it impossible for a concrete struct to
implement a generic trait.

The new syntax looks like a trait impl `impl X for Y`.  That solves all
the problems with generics.  In addition, it allows `mock!` to implement
traits that aren't imported directly into the current namespace
`impl x::y::Z for Foo`.

The old syntax is deprecated, but will be supported for at least one
more release.

Fixes #119