Runtime: use monomorphic representation for MlBytes

Author: hhugo

CHANGES.md

@@ -25,6 +25,7 @@
   requested at compile time (--enable with-js-error) or at startup (OCAMLRUNPARAM=b=1)
 * Runtime: allow dynlink of precompiled js with separate compilation (#1676)
 * Runtime: reimplement the runtime of weak and ephemeron (#1707)
+* Runtime: refactor mlBytes
 * Lib: Modify Typed_array API for compatibility with WebAssembly
 * Toplevel: no longer set globals for toplevel initialization