Cleanup and make `BitWidth` use `NonZeroUsize`

[AaronKutch]

I removed the dependency on specialized-div-rem because I plan on finishing my PR to compiler-builtins in the next few weeks. I updated the smallvec dependency because it did not allow compilation on stable with --no-default-features. smallvec is no-std by default now. Imports were added to mem. Even though they are not used much now, it will save a lot when most files need a TryFrom import in later PRs. I did some other random cleanup, and no warnings are issued with any combination of feature flags on stable now (except for clippy warnings and TODO warnings that will be fixed down the road).

[AaronKutch]

Do you want me to introduce the bw function next? Removing the impl From<usize> for BitWidth impl causes a cascade of changes I have to make all at once or else the crate is completely broken, but I realized I can do the bw changes standalone.

[coveralls]

Coverage decreased (-0.8%) to 81.145% when pulling 5a81230d7f4984b01db889a08510410f1c9b9333 on AaronKutch:issue-41-nonzerousize into 2818e58c779de645aab7fd6243c5fe8d0229f292 on Robbepop:master.

[codecov-commenter]

Codecov Report

Merging #59 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #59   +/-   ##
=======================================
  Coverage   81.87%   81.87%           
=======================================
  Files          23       23           
  Lines        5132     5132           
=======================================
  Hits         4202     4202           
  Misses        930      930           

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[AaronKutch]

I already have the next PR ready, I'm just waiting for this to be merged (The PRs that effect the entire crate cannot be done in parallel without commit dependency conflict problems).

[Robbepop]

I already have the next PR ready, I'm just waiting for this to be merged (The PRs that effect the entire crate cannot be done in parallel without commit dependency conflict problems).

merged