Allow for implementations on generic structs

[jamesgarfield]

Allow for pattern matching on struct types.

From a generic specification of

type T struct {
    done chan struct{}
}

func (t T) Terminate() {
    t.done <- struct{}{}
}

be able to pattern match onto structurally similar, but extended structs, e.g.

type IntStep {
    In   chan <- int
    Out  <- chan int
    done chan struct{}
}

This naturally leads to generic specifications on empty structs being applicable to all structs.

Possible application

type T struct{}
func (t T) Json() string {
   //generic json encoding code
}