feat: expose (utf8|binary)_substring

[orlp]

Closes #1538.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (36e905d) 83.45% compared to head (d9c4b38) 83.45%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1539   +/-   ##
=======================================
  Coverage   83.45%   83.45%           
=======================================
  Files         388      388           
  Lines       42138    42142    +4     
=======================================
+ Hits        35165    35171    +6     
+ Misses       6973     6971    -2     

Files Changed	Coverage Δ
src/compute/substring.rs	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[ritchie46]

Can you run a cargo fmt?

[orlp]

Oops, sorry. Done.