Allow top-level string literals in Core (#8472)

This commits relaxes the invariants of the Core syntax so that a
top-level variable can be bound to a primitive string literal of type
Addr#.

This commit:

* Relaxes the invatiants of the Core, and allows top-level bindings whose
  type is Addr# as long as their RHS is either a primitive string literal or
  another variable.

* Allows the simplifier and the full-laziness transformer to float out
  primitive string literals to the top leve.

* Introduces the new StgGenTopBinding type to accomodate top-level Addr#
  bindings.

* Introduces a new type of labels in the object code, with the suffix "_bytes",
  for exported top-level Addr# bindings.

* Makes some built-in rules more robust. This was necessary to keep them
  functional after the above changes.

This is a continuation of D2554.

Rebasing notes:
This had two slightly suspicious performance regressions:

* T12425: bytes allocated regressed by roughly 5%
* T4029: bytes allocated regressed by a bit over 1%
* T13035: bytes allocated regressed by a bit over 5%

These deserve additional investigation.

Rebased by: bgamari.

Test Plan: ./validate --slow

Reviewers: goldfire, trofi, simonmar, simonpj, austin, hvr, bgamari

Reviewed By: trofi, simonpj, bgamari

Subscribers: trofi, simonpj, gridaphobe, thomie

Differential Revision: https://phabricator.haskell.org/D2605

GHC Trac Issues: #8472

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