crypto: split up the machine.rs behemoth

[richvdh]

The machine.rs file has become increasingly unwieldy, and hard to navigate. In particular, it's hard to figure out what we have in the way of tests, because there are just 3000 lines of rather opaque tests.

By way of trying to improve the situation, I've split the tests out to a separate file, and thence tried to extract some themes. It's slightly arbitrary, but maybe better than nothing.

I'm not sure if this is going to be reviewable in the conventional manner :/.

[codecov]

Codecov Report

Attention: Patch coverage is 85.63135% with 99 lines in your changes missing coverage. Please review.

Project coverage is 84.12%. Comparing base (f3fe06b) to head (045374c).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/matrix-sdk-crypto/src/machine/mod.rs	85.63%	99 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3813      +/-   ##
==========================================
- Coverage   84.15%   84.12%   -0.03%     
==========================================
  Files         263      263              
  Lines       27584    27584              
==========================================
- Hits        23213    23206       -7     
- Misses       4371     4378       +7     

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