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/ext.go

@@ -0,0 +1,9 @@
+// Package ext is used as a target package in the external types example.
+package ext
+
+// Struct is the target type in the external types example.
+type Struct struct {
+	A [3]uint16
+	B byte
+	C string
+}