Implement local devirtualization

[quasilyte]

Go compiler rarely does a good job at removing the interfaces overhead.
Here is an example:

func f(buf *bytes.Buffer) {
  var rw io.ReadWriter = b
  g(rw) // performs convI2I
}

//go:noinline
func g(r io.Reader) {}

Fixed code:

func f(buf *bytes.Buffer) {
  var rw = b // just use *bytes.Buffer directly
  g(rw) // cheaper conversion
}

//go:noinline
func g(r io.Reader) {}

Investigation needed: how often can we improve things?