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

build/context.go

@@ -5,6 +5,7 @@ import (
 	"go/types"
 
 	"github.com/mmcloughlin/avo"
+	"github.com/mmcloughlin/avo/attr"
 	"github.com/mmcloughlin/avo/buildtags"
 	"github.com/mmcloughlin/avo/gotypes"
 	"github.com/mmcloughlin/avo/operand"
@@ -90,7 +91,7 @@ func (c *Context) Doc(lines ...string) {
 }
 
 // Attributes sets function attributes for the currently active function.
-func (c *Context) Attributes(a avo.Attribute) {
+func (c *Context) Attributes(a attr.Attribute) {
 	c.activefunc().Attributes = a
 }
 
@@ -124,12 +125,12 @@ func (c *Context) AllocLocal(size int) operand.Mem {
 
 // Instruction adds an instruction to the active function.
 func (c *Context) Instruction(i *avo.Instruction) {
-	c.activefunc().AddNode(i)
+	c.activefunc().AddInstruction(i)
 }
 
 // Label adds a label to the active function.
-func (c *Context) Label(l avo.Label) {
-	c.activefunc().AddLabel(l)
+func (c *Context) Label(name string) {
+	c.activefunc().AddLabel(avo.Label(name))
 }
 
 func (c *Context) activefunc() *avo.Function {
@@ -150,7 +151,7 @@ func (c *Context) StaticGlobal(name string) operand.Mem {
 }
 
 // DataAttributes sets the attributes on the current active global data section.
-func (c *Context) DataAttributes(a avo.Attribute) {
+func (c *Context) DataAttributes(a attr.Attribute) {
 	c.activeglobal().Attributes = a
 }