gotypes,build: add Implement (#58)

By using Implement you can provide a definition of a function, taking the signature from a stub in the package. One major benefit of this approach is it makes it easy to handle external types in the function signature.

Updates #55

examples/ext/ext_test.go

@@ -0,0 +1,17 @@
+package ext
+
+import (
+	"testing"
+	"testing/quick"
+
+	"github.com/mmcloughlin/avo/examples/ext/ext"
+)
+
+//go:generate go run asm.go -out ext.s
+
+func TestFunc(t *testing.T) {
+	expect := func(e ext.Struct) byte { return e.B }
+	if err := quick.CheckEqual(StructFieldB, expect, nil); err != nil {
+		t.Fatal(err)
+	}
+}