Update compact_str to v0.4

[ParkMyCar]

Context In the most recent version of compact_str, we renamed CompactStr to CompactString. You can continue to use CompactStr but there is a deprecation warning on it.

Changes This PR updates compact_str to v0.4, renaming uses of CompactStr to CompactString to prevent the warning

[codecov[bot]]

Codecov Report

Merging #9 (c29b731) into master (c1969df) will decrease coverage by 39.30%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master       #9       +/-   ##
===========================================
- Coverage   84.31%   45.01%   -39.31%     
===========================================
  Files           8        8               
  Lines         459      451        -8     
===========================================
- Hits          387      203      -184     
- Misses         72      248      +176     

Impacted Files	Coverage Δ
flexstr/src/traits.rs	13.86% <0.00%> (-85.15%)	:arrow_down:
flexstr/src/lib.rs	35.08% <0.00%> (-46.74%)	:arrow_down:
flexstr/src/impls.rs	39.28% <0.00%> (-32.48%)	:arrow_down:
flexstr/src/builder.rs	82.19% <0.00%> (-13.70%)	:arrow_down:
flexstr/src/storage/inline.rs	79.36% <0.00%> (-1.59%)	:arrow_down:

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[nu11ptr]

No issue pulling this in, but for some weird reason my code coverage run is now saying this reduces it by 50% - obviously not true, but will have to figure out why it is doing that.