Wrapper struct for models of a theory

[kris-brown]

This addresses #178

One can declare that Foo is a single-field struct which wraps models of ThCategory via ThCategory.Meta.@wrapper Foo. This type can be a subtype of an abstract type too: ThCategory.Meta.@wrapper Foo <: MyAbsType.

We may also want to generate a wrapper struct which has type parameters for the associated Julia types, so ThCategory.Meta.@typed_wrapper Foo means that Foo(FinCatC()) isa Foo{Int,Vector{Int}}

It's unclear if we should just have the typed version and, if one doesn't need to use the type parameters, one can ignore them.

[codecov]

Codecov Report

Attention: Patch coverage is 98.07692% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.09%. Comparing base (69dc438) to head (cc44137).

Files with missing lines	Patch %	Lines
src/syntax/TheoryInterface.jl	98.00%	1 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                     @@
##           symbolic_model_explicit     #180      +/-   ##
===========================================================
+ Coverage                    95.04%   95.09%   +0.05%     
===========================================================
  Files                           38       38              
  Lines                         2299     2345      +46     
===========================================================
+ Hits                          2185     2230      +45     
- Misses                         114      115       +1     

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