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/README.md

@@ -0,0 +1,33 @@
+# ext
+
+Demonstrates how to use external types in an `avo` function signature.
+
+In this case, you will need to write the function stub yourself.
+
+[embedmd]:# (stub.go /package/ $)
+```go
+package ext
+
+import "github.com/mmcloughlin/avo/examples/ext/ext"
+
+// StructFieldB returns field B.
+func StructFieldB(e ext.Struct) byte
+```
+
+Then in place of the usual `TEXT` declaration we use `Implement` to state that we are beginning the definition of a function already declared in the package stub file.
+
+[embedmd]:# (asm.go go /.*Package.*/ /RET.*/)
+```go
+	Package("github.com/mmcloughlin/avo/examples/ext")
+	Implement("StructFieldB")
+	b := Load(Param("e").Field("B"), GP8())
+	Store(b, ReturnIndex(0))
+	RET()
+```
+
+Finally, in this case the `go:generate` line is different since we do not need to generate the stub file.
+
+[embedmd]:# (ext_test.go go /.*go:generate.*/)
+```go
+//go:generate go run asm.go -out ext.s
+```