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

cakeml/basis/pure/mlprettyprinterScript.sml

Lines 217 to 219 in 5204d42

	Definition pp_ref_def:
	pp_ref f r = pp_token (strlit "<ref>")
	End

This should be what's printed instead

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