Support String patterns

[AntonOresten]

Currently has deprecation warnings to warn when these are actually used. It's only shown when running Julia with --depwarn=yes though. Might need to pass force=true since people are most likely to use it in REPL, or accidentally in source code.

Closes #42

[codecov]

Codecov Report

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.97%. Comparing base (8a848de) to head (726afcf).

Files with missing lines	Patch %	Lines
src/String.jl	0.00%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   98.69%   94.97%   -3.72%     
==========================================
  Files          12       13       +1     
  Lines         383      398      +15     
==========================================
  Hits          378      378              
- Misses          5       20      +15     

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[AntonOresten]

Note that this is not and can't be done for reshape (#50), reduce, or repeat due to type piracy as they are defined in Base.

[AntonOresten]

A possible workaround is defining the functions in Einops, such that users need to do using Einops: reshape, reduce, repeat, dispatching to Base.reshape when it doesn't match to ArrowPattern or String. This is kind of an all-or-nothing, where reduce can only be implicitly imported by one module, and it already is from Base.

[AntonOresten]

I would really like to find a way to constant-propagate String. ccall has some String argument but I think that's a special case and not a normal function since it can't be taken as a variable.