In a previous patch we replaced some built-in literal constructors
(MachInt, MachWord, etc.) with a single LitNumber constructor.
In this patch we replace the Mach prefix of the remaining constructors
with Lit for consistency (e.g., LitChar, LitLabel, etc.).
Sadly the name LitString was already taken for a kind of FastString
and it would become misleading to have both LitStr (literal
constructor renamed after MachStr) and LitString (FastString
variant). Hence this patch renames the FastString variant PtrString
(which is more accurate) and the literal string constructor now uses the
least surprising LitString name.
Both Literal and LitString/PtrString have recently seen breaking
changes so doing this kind of renaming now shouldn't harm much.