Allow checking if a model does NOT implement a given theory

[kris-brown]

Addresses #174

This fixes the typo of the catch-all case, i.e.

implements(m::Module,::Type{Val{tag}}) where tag = nothing

We replace Module with Any.

However, the way we check whether or not an @instance call needs to define a new instance method is whether or not there are any methods implements(::ModelType, Val{specific_tag}). So now there is one that is always there by default.

One would think, then, that the correct way to fix this logic is to check whether or not there is exactly one method, rather than zero like before. However, because of how methods works, once some earlier @instance call has already defined the more specific implements method (which returns not nothing), it's not that there are now two methods but there still is just one (it's the newer, more specific one). So the logic check is to simply check whether or not the only method is equal to the catch-all case.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.48%. Comparing base (86e6d51) to head (3d4bf2c).

Additional details and impacted files

@@              Coverage Diff              @@
##           rem_model     #175      +/-   ##
=============================================
+ Coverage      95.43%   95.48%   +0.04%     
=============================================
  Files             38       38              
  Lines           2257     2258       +1     
=============================================
+ Hits            2154     2156       +2     
+ Misses           103      102       -1     

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