Fixes #401

that performs `wp_pure` in small steps until the lemma matches the goal.

Tej Chajed authored 4 years ago and Robbert Krebbers committed 4 years ago

Addresses #315 for several tactics (`wp_alloc`, `wp_load`, `wp_store`,
`wp_free`).

This is more consistent with other tac_wp tactics for HeapLang and also
a tiny bit more efficient, which is good to embody in the basic tactics
so derived code follows the same patterns.

- Export `total_weakestpre` in `lifting` already, like we do for `weakestpre`.
- Do not export modules that are already exported by exported modules.

The general idea is to first import/export modules which are further
than the current one, and then import/export modules which are close
dependencies.

This commit tries to use the same order of imports for every file, and
describes the convention in ProofGuide.md. There is one exception,
where we do not follow said convention: in program_logic/weakestpre.v,
using that order would break printing of texan triples (??).

This also gets rid of [val_for_compare]-normalization; instead we introduce a
[LitErased] literal that is suited for use by erasure theorems.

This is the name used by x86 (CMPXCHG) and LLVM.  Also reorder the result (value
first, boolean second) for consistency with LLVM, because why not.