Remove "no_std" + "nightly" features and support no_std on stable

[brycx]

subtle has added optimization barriers on stable Rust, in version 2.2.1. The reason we had a nightly feature before was to take advantage of these barriers, that were only available on nightly at the time.

This PR removes both no_std and nightly features in orion, making the crate-feature story a lot more simple. This enables using orion with no_std on both stable and nightly Rust (nightly is required for no_std now), only needing to specify orion = { version = "*", default-features = false }.

[codecov]

Codecov Report

Merging #111 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
+ Coverage   97.37%   97.45%   +0.08%     
==========================================
  Files          48       48              
  Lines        6168     6168              
==========================================
+ Hits         6006     6011       +5     
+ Misses        162      157       -5

Impacted Files	Coverage Δ
src/hazardous/stream/xchacha20.rs	92.85% <0%> (-1.43%)	⬇️
src/hazardous/kdf/hkdf.rs	98.83% <0%> (-1.17%)	⬇️
src/hazardous/aead/streaming.rs	98.42% <0%> (+0.19%)	⬆️
src/hazardous/stream/chacha20.rs	95.91% <0%> (+0.27%)	⬆️
src/aead.rs	100% <0%> (+1.2%)	⬆️
src/auth.rs	100% <0%> (+1.81%)	⬆️
src/kdf.rs	100% <0%> (+2.85%)	⬆️

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