"no-std" categories in "Cargo.toml"

[Philippe-Cholet]

This crate can be used in no-std context, with alloc or not.
So I add no-std and no-std::no-alloc to the categories in Cargo.toml.

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I checked what is done in similar conditions and found that serde have some similar features

[features] default = ["std"] std = [] alloc = [] 

And has these two categories as well: https://github.com/serde-rs/serde/blob/00c4b0cef80557c33fbcd75fcc70dc034720b4df/serde/Cargo.toml#L6

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.44%. Comparing base (6814180) to head (73c641a).
Report is 30 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #894 +/- ## ========================================== + Coverage 94.38% 94.44% +0.05%  ========================================== Files 48 48 Lines 6665 6749 +84 ========================================== + Hits 6291 6374 +83  - Misses 374 375 +1 

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