build: unify Label function signatures

The Context.Label method and LABEL global function did not agree. Also
breaks the convention I'd like to set that capitalized functions must
agree with existing Go assembly syntax.

To help avoid a conflict with `avo.Label`, attributes were moved to
their own package.

Fixes #35

ast.go

@@ -3,6 +3,7 @@ package avo
 import (
 	"errors"
 
+	"github.com/mmcloughlin/avo/attr"
 	"github.com/mmcloughlin/avo/buildtags"
 	"github.com/mmcloughlin/avo/gotypes"
 	"github.com/mmcloughlin/avo/operand"
@@ -128,7 +129,7 @@ func (f *File) Functions() []*Function {
 // Function represents an assembly function.
 type Function struct {
 	Name       string
-	Attributes Attribute
+	Attributes attr.Attribute
 	Doc        []string
 	Signature  *gotypes.Signature
 	LocalSize  int
@@ -236,7 +237,7 @@ func (d Datum) Overlaps(other Datum) bool {
 // Global represents a DATA section.
 type Global struct {
 	Symbol     operand.Symbol
-	Attributes Attribute
+	Attributes attr.Attribute
 	Data       []Datum
 	Size       int
 }