feat: fix and test for no_std with alloc

[tommilligan]

For #82

Work in progress towards no_std support (will still require alloc). Things still to do:

• readme
• changelog
• decide what version this should be, feels like a good excuse for v1
• benchmark? should be identical

[codecov-commenter]

Codecov Report

Merging #83 (659b66b) into main (9ef3e0d) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   96.95%   96.85%   -0.10%     
==========================================
  Files           4        4              
  Lines         197      191       -6     
==========================================
- Hits          191      185       -6     
  Misses          6        6              

Impacted Files	Coverage Δ
pretty_assertions/src/lib.rs	100.00% <ø> (ø)
pretty_assertions/src/printer.rs	99.24% <ø> (ø)
pretty_assertions/tests/macros.rs	97.95% <ø> (-0.23%)	⬇️

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