cfi: various cleanups

[philipc]

Try to simplify and makes things more consistent. I was looking into this as part of seeing if we could do more for #296, but I didn't come up with anything significant.

The commit I'm most unsure about is eeae767 (Change callbacks to match UnwindSection::cie_from_offset).

There are minor performance improvements:

 name                                                       before ns/iter  after ns/iter  diff ns/iter  diff %  speedup 
 cfi::eval_longest_fde_instructions_new_ctx_everytime       532             546                      14   2.63%   x 0.97 
 cfi::eval_longest_fde_instructions_same_ctx                353             329                     -24  -6.80%   x 1.07 
 cfi::iterate_entries_and_do_not_parse_any_fde              63,038          57,991               -5,047  -8.01%   x 1.09 
 cfi::iterate_entries_and_parse_every_fde                   490,942         450,314             -40,628  -8.28%   x 1.09 
 cfi::iterate_entries_and_parse_every_fde_and_instructions  1,065,654       1,013,824           -51,830  -4.86%   x 1.05 
 cfi::iterate_entries_evaluate_every_fde                    1,376,872       1,327,763           -49,109  -3.57%   x 1.04 
 cfi::parse_longest_fde_instructions                        224             222                      -2  -0.89%   x 1.01 

[philipc]

Another thing to consider is the use of Box for UninitializedUnwindContext. Since we now only ever accept it by reference, and don't hold long lived references to it (only in UnwindTable which is normally short lived), maybe we should remove the Box now? Then it would be up to the caller to decide where they want to allocate it.

[coveralls]

Coverage increased (+0.009%) to 86.623% when pulling bfc4f72 on philipc:issue-296-2 into b8d7ee7 on gimli-rs:master.

[fitzgen]

Another thing to consider is the use of Box for UninitializedUnwindContext. Since we now only ever accept it by reference, and don't hold long lived references to it (only in UnwindTable which is normally short lived), maybe we should remove the Box now? Then it would be up to the caller to decide where they want to allocate it.

The original motivation for the box was to avoid moving large structs around on the stack, if I recall correctly. Now that we have switched to a reference-based API instead of a move-based API, this seems like a good change to make.

[fitzgen]

👍