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
+```

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()
+}

examples/ext/ext.s

@@ -0,0 +1,7 @@
+// Code generated by command: go run asm.go -out ext.s. DO NOT EDIT.
+
+// func StructFieldB(e ext.Struct) byte
+TEXT ·StructFieldB(SB), $0-25
+	MOVB e_B+6(FP), AL
+	MOVB AL, ret+24(FP)
+	RET

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
+}

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)
+	}
+}

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