Add #![no_std] support

[FreezyLemon]

Closes #165.

Standard library types that exist in libcore are usually just reexported through std::, so changing from e.g. std::f64 to core::f64 results in no change in behavior.

There are a few things to note:

• All nalgebra-dependent features require std at the moment. I'm not sure OVector/OMatrix can be used without std because of Dyn, but nalgebra does support no_std and SVector/SMatrix should work without libstd. Out of scope here though.
• Some other features also require libstd, so they're disabled in no_std. E.g. all functions that have a Vec parameter/return type like OrderStatistics::ranks.
• core::error::Error exists, but was only recently stabilized in 1.81.0. I decided to not use this yet and instead not implement the Error trait in no_std.
• Tests have worse error messages in no_std. I don't know how to this type of string formatting without String, so I just dropped it. Dev machines should usually have it available anyways.

[codecov]

Codecov Report

Attention: Patch coverage is 41.09589% with 43 lines in your changes missing coverage. Please review.

Project coverage is 93.81%. Comparing base (a8fe65c) to head (2553589).
Report is 35 commits behind head on master.

Files with missing lines	Patch %	Lines
src/distribution/internal.rs	0.00%	5 Missing ⚠️
src/generate.rs	16.66%	5 Missing ⚠️
src/distribution/bernoulli.rs	0.00%	1 Missing ⚠️
src/distribution/beta.rs	0.00%	1 Missing ⚠️
src/distribution/binomial.rs	0.00%	1 Missing ⚠️
src/distribution/categorical.rs	0.00%	1 Missing ⚠️
src/distribution/cauchy.rs	0.00%	1 Missing ⚠️
src/distribution/chi.rs	0.00%	1 Missing ⚠️
src/distribution/chi_squared.rs	0.00%	1 Missing ⚠️
src/distribution/dirac.rs	0.00%	1 Missing ⚠️
... and 25 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
- Coverage   93.81%   93.81%   -0.01%     
==========================================
  Files          53       53              
  Lines       11996    12005       +9     
==========================================
+ Hits        11254    11262       +8     
- Misses        742      743       +1     

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[YeungOnion]

I forgot why I didn't merge this earlier, but I also see a few items we can add for future development,

• empirical as an immutable distribution (I've already been considering this since it's the only distribution with this behavior), an array would be sorted in place, then a second array would be buffered to the same size and filled with the values and cumulative counts.
• OrderStatistic could have an API that outputs iterables instead of explicit Vec, this also means that if I only wanted to know where in a sorted list the first k items in an unsorted list appears, I could have the benefit of only obtaining the rank of the first three values, while worst case is unchanged, I think typical case would improve meaningfully for larger datasets.

Thanks for doing this, I think since I took a while with this there may be more changes required and then we'll have to update PRs.

[FreezyLemon]

Quite a few things were added between when I opened this and now.. this probably should've been updated before merging, as e.g. no-default-features is now broken on master. I'll try to fix those and open a new PR.