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/asm.go

@@ -0,0 +1,16 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+)
+
+func main() {
+	Package("github.com/mmcloughlin/avo/examples/ext")
+	Implement("StructFieldB")
+	b := Load(Param("e").Field("B"), GP8())
+	Store(b, ReturnIndex(0))
+	RET()
+	Generate()
+}