Add no_std support

[C0D3-M4513R]

(cherry picked from commit 191d4f4)

Summary of the PR

Please summarize here why the changes in this PR are needed.

It makes this crate actually usable in no_std environments.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR are signed (with git commit -s), and the commit
  message has max 60 characters for the summary and max 75 characters for each
  description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[C0D3-M4513R]

Refusing to touch the Changelog, since:

You don't need to edit the changelog it is automatically generated from commit messages.

The error impls are not from core, because: rust-lang/rust#103765
That's also the same reason, why std enabled by default.

[codecov]

Codecov Report

Attention: Patch coverage is 94.63087% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 90.35%. Comparing base (1a64e5d) to head (e73c90e).
Report is 5 commits behind head on master.

Files	Patch %	Lines
ux2-macros/src/lib.rs	95.91%	6 Missing ⚠️
ux2/src/lib.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   90.17%   90.35%   +0.17%     
==========================================
  Files           2        2              
  Lines        1547     1555       +8     
==========================================
+ Hits         1395     1405      +10     
+ Misses        152      150       -2     

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[JonathanWoollett-Light]

Could you please remove the commit message line

(cherry picked from commit 191d4f4)

from the commit.

[C0D3-M4513R]

Done

[JonathanWoollett-Light]

lgtm