Make the crate no_std compatible?

[JSorngard]

I may have figured out a way to make the crate no_std compatible!

I added the std feature (disabled by default currently) that can be activated to enable the current Backtrace functionality of the error types, since that needs the standard library.

Potential trade-offs:

• rayon depends on the standard library, so if that feature is enabled it doesn't really matter that we are no_std.
• Depend on the libm crate to compute square roots and trigonometric functions if the std feature is disabled. When the std feature is enabled we depend on libm even if we never use it.
• Increases the minimum compatible Rust version to 1.81 since we then use the Error trait in core.

Roadblocks

Currently the compiler complains when I run cargo check that there is no allocator, but it works when I run cargo test. I do not know how to resolve this yet. I am also confused about why it doesn't work right now, as I have already used the functionality I use here to make my const-primes crate no_std, and that worked without this issue.

[codecov]

Codecov Report

Attention: Patch coverage is 91.95402% with 7 lines in your changes missing coverage. Please review.

Project coverage is 94.77%. Comparing base (41f0172) to head (5187bb8).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/data_api.rs	70.00%	3 Missing ⚠️
src/hermite/mod.rs	87.50%	1 Missing ⚠️
src/legendre/mod.rs	80.00%	1 Missing ⚠️
src/midpoint/mod.rs	80.00%	1 Missing ⚠️
src/simpson/mod.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   94.78%   94.77%   -0.02%     
==========================================
  Files           9       10       +1     
  Lines        1689     1742      +53     
==========================================
+ Hits         1601     1651      +50     
- Misses         88       91       +3     

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