1.0.29.54: Inline unboxed constants on x86[-64]

1.0.29.54: Inline unboxed constants on x86[-64]

* New build-time feature: inline-constants, which specifies that SB!C
  and SB!VM implement a protocol described in base-target-features.lisp-expr.
  Backends implementing that feature are able to load constants from code
  components, in a section that follows the actual executable code.

* Implement the protocol on x86 and x86-64, and use it for float constants,
  and, on x86-64 only, mid-sized (> 2^(29-32), but still machine-sized)
  integers.

* Use the new feature in integer and float arithmetic VOPs.

* Adjust a few test cases to take newly consing situations into account.

* Clean-up:
  - New build-time feature: float-eql-vops, which disable rewriting EQL
    of single and double floats in terms of foo-float*-bits.
  - Fix a typo (unused variable lookup) in TWO-ARG-+/-