Add stateful pretty printer for references in REPL

[ordinarymath]

When using the repl References are printed as <ref>

> val a = Ref 0;
val a = <ref>: int ref

Current implementation is here https://github.com/CakeML/cakeml/blob/5204d423c1b89d92a970f41f52da476d6cfb1e47/basis/pure/mlprettyprinterScript.sml#L217-L219 This should be what's printed instead

> val a = Ref 0;
val a = Ref 0: int ref