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.

Reviewers: hvr, goldfire, bgamari, simonmar, jrtc27, tdammers

Subscribers: tdammers, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4881