equihash: Fix some Rust API issues

[teor2345]

This is a fully-functional draft, but it contains some extra lint handling and review comments.

[teor2345]

Here are the major changes I've made to the code. The solution allocation type fix is required, everything else is optional.

(There are also a bunch of lints and comments that can be ignored, I'll clean them up when we get a valid solution from the code.)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f3ea80e) 66.52% compared to head (aff6fac) 66.71%.

Files	Patch %	Lines
components/equihash/src/tromp.rs	98.61%	1 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##           equihash-solver-tromp    #1088      +/-   ##
=========================================================
+ Coverage                  66.52%   66.71%   +0.18%     
=========================================================
  Files                        115      115              
  Lines                      10881    10943      +62     
=========================================================
+ Hits                        7239     7301      +62     
  Misses                      3642     3642              

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[teor2345]

It looks like Windows and macOS need a portable endian.h, so I added a trivial permissively licensed header.

Also Windows didn't like the thread support, which we're not using anyway, so I deleted it.

[teor2345]

Since we're not using threads, we could remove id from all these functions and replace it with zero in C.

[str4d]

Changes generally LGTM. I'm going to merge this and then do some refactoring / squashing in #1083, to in particular ensure that this is also usable by zcashd (so we can deduplicate the code therein).