Removed `index_to_bitset` from the dataflow context.

Part of addressing 15019.
Removed the `_frozen` methods from dataflow API.

Calls to methods like `each_bit_on_entry_frozen` and
`each_gen_bit_frozen` now go to the `each_bit_on_entry` and
`each_gen_bit` methods.
librustc: Implement unboxed closures with mutable receivers
auto merge of #14539 : pcwalton/rust/unboxed-closures, r=pnkfelix

@nikomatsakis r?
auto merge of #15742 : pnkfelix/rust/fsk-fix-15019, r=pcwalton

Removed `index_to_bitset` field and `_frozen` methods.

Drive-by: Added some missing docs on the `each_bit` method.

Drive-by: Put in a regular pattern: when calling `compute_id_range`, ensure `words_per_id > 0` by either asserting it or checking and returning early.  (The prior code did the latter in a few cases where necessary, but debugging is much aided by the asserts.)

Fix #15019.
Add support for patterns referencing non-trivial statics

This is accomplished by rewriting static expressions into equivalent patterns.
This way, patterns referencing static variables can both participate
in exhaustiveness analysis as well as be compiled down into the appropriate
branch of the decision trees that match expressions are codegened to.

Fixes #6533.
Fixes #13626.
Fixes #13731.
Fixes #14576.
Fixes #15393.