Simple theory maps and simple pushouts

[kris-brown]

This adds a new instance of AbsTheoryMap which maps generators to generators, rather than to complex terms. For this kind of map we can easily compute pushouts, which is a matter of renaming.

The universal property of these colimits is not yet implemented.

Also, this is all at the GAT data structure level, rather than the module level.

[codecov]

Codecov Report

Attention: Patch coverage is 96.96970% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 94.80%. Comparing base (53c6086) to head (a65ffdf).

Files	Patch %	Lines
src/syntax/TheoryMaps.jl	94.33%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   94.52%   94.80%   +0.27%     
==========================================
  Files          38       38              
  Lines        2082     2174      +92     
==========================================
+ Hits         1968     2061      +93     
+ Misses        114      113       -1     

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[olynch]

My thought is that we are going to use the multiple-inheritance branch instead of this, so this should not be merged, is that right @kris-brown?

[kris-brown]

I imagined them as two different means of doing the same thing - I'd be happy to re-implement this down the line (in case we discover a need for programmatically merging theories), using the underlying implementation in multiple-inheritance, so I think it's fine to put this branch on pause / close the PR for now.