interp: implement runtime.sliceCopy

This implements the copy() built-in function. It may not work in all
cases, but should work in most cases.

This commit gets the following 3 packages to compile, according to
tinygo-site/imports/main.go:

  * encoding/base32
  * encoding/base64
  * encoding/pem (was blocked by encoding/base64)

interp/interp_test.go

@@ -12,6 +12,7 @@ import (
 func TestInterp(t *testing.T) {
 	for _, name := range []string{
 		"basic",
+		"slice-copy",
 	} {
 		name := name // make tc local to this closure
 		t.Run(name, func(t *testing.T) {