"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.

---

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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