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/stub.go

@@ -0,0 +1,7 @@
+// Package ext demonstrates how to interact with external types.
+package ext
+
+import "github.com/mmcloughlin/avo/examples/ext/ext"
+
+// StructFieldB returns field B.
+func StructFieldB(e ext.Struct) byte