chore: clippy cleanup

[rakita]

Clippy got a few new things in new nightly.

switched CI to stable, and make alloc imports be std::

[github-actions]

Valgrind Results:

==3981== Cachegrind, a cache and branch-prediction profiler
==3981== Copyright (C) 2002-2017, and GNU GPL'd, by Nicholas Nethercote et al.
==3981== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==3981== Command: target/release/snailtracer
==3981== 
--3981-- warning: L3 cache found, using its data for the LL simulation.
Snailtracer Host+Interpreter benchmark (2.1s) ...            not enough samples
==3981== 
==3981== I   refs:      471,785,703
==3981== I1  misses:          5,856
==3981== LLi misses:          2,904
==3981== I1  miss rate:        0.00%
==3981== LLi miss rate:        0.00%
==3981== 
==3981== D   refs:      229,164,274  (152,345,985 rd   + 76,818,289 wr)
==3981== D1  misses:        350,871  (    210,583 rd   +    140,288 wr)
==3981== LLd misses:        137,578  (      4,054 rd   +    133,524 wr)
==3981== D1  miss rate:         0.2% (        0.1%     +        0.2%  )
==3981== LLd miss rate:         0.1% (        0.0%     +        0.2%  )
==3981== 
==3981== LL refs:           356,727  (    216,439 rd   +    140,288 wr)
==3981== LL misses:         140,482  (      6,958 rd   +    133,524 wr)
==3981== LL miss rate:          0.0% (        0.0%     +        0.2%  )