ir,build: pragma support (#97)

Adds support for arbitrary compiler directives.

Fixes #15

examples/pragma/README.md

@@ -0,0 +1,33 @@
+# pragma
+
+Apply [compiler directives](https://golang.org/pkg/cmd/compile/#hdr-Compiler_Directives) to `avo` functions.
+
+The [code generator](asm.go) uses the `Pragma` function to apply the `//go:noescape` directive to the `Add` function:
+
+[embedmd]:# (asm.go go /func main/ /^}/)
+```go
+func main() {
+	TEXT("Add", NOSPLIT, "func(z, x, y *uint64)")
+	Pragma("noescape")
+	Doc("Add adds the values at x and y and writes the result to z.")
+	zptr := Mem{Base: Load(Param("z"), GP64())}
+	xptr := Mem{Base: Load(Param("x"), GP64())}
+	yptr := Mem{Base: Load(Param("y"), GP64())}
+	x, y := GP64(), GP64()
+	MOVQ(xptr, x)
+	MOVQ(yptr, y)
+	ADDQ(x, y)
+	MOVQ(y, zptr)
+	RET()
+	Generate()
+}
+```
+
+Note the directive is applied in the generated stub file:
+
+[embedmd]:# (stub.go go /\/\/ Add/ /func/)
+```go
+// Add adds the values at x and y and writes the result to z.
+//go:noescape
+func
+```