To enable the usage of the with statement for the Solver class
instances, this commit adds __enter__ and __exit__ methods.
The with statement should offer a more concise and safer
alternative to explicit usage of push() and pop() methods.
`FindPythonInterp` has been deprecated for a long time and is more
verbal about that deprecation now.
The build system no longer uses `PYTHON_EXECUTABLE` but instead uses
`Python3_EXECUTABLE`.
An initial update to support polymorphism from SMTLIB3 and the API (so far C, Python).
The WIP SMTLIB3 format is assumed to be supporting the following declaration
```
(declare-type-var A)
```
Whenever A is used in a type signature of a function/constant or bound quantified variable, it is taken to mean that all instantiations of A are included in the signature and assertions.
For example, if the function f is declared with signature A -> A, then there is a version of f for all instances of A.
The semantics of polymorphism appears to follow previous proposals: the instances are effectively different functions.
This may clash with some other notions, such as the type signature forall 'a . 'a -> 'a would be inhabited by a unique function (the identity), while this is not enforced in this version (and hopefully never because it is more busy work).
The C API has the function 'Z3_mk_type_variable' to create a type variable and applying functions modulo polymorphic type signatures is possible.
The kind Z3_TYPE_VAR is added to sort discriminators.
This version is considered as early alpha. It passes a first rudimentary unit test involving quantified axioms, declare-fun, define-fun, and define-fun-rec.
This update includes an experimental feature to access a congruence closure data-structure after search.
It comes with several caveats as pre-processing is free to eliminate terms. It is therefore necessary to use a solver that does not eliminate the terms you want to track for congruence of. This is partially addressed by using SimpleSolver or incremental mode solving.
```python
from z3 import *
s = SimpleSolver()
x, y, z = Ints('x y z')
s.add(x == y)
s.add(y == z)
s.check()
print(s.root(x), s.root(y), s.root(z))
print(s.next(x), s.next(y), s.next(z))
```
This update allows the python bindings for user-propagator to handle functions that are declared to be registered with the user propagator plugin. It fixes a bug in UserPropagateBase.add to allow registering terms dynamically during search.
It also fixes a bug in theory_user_propagate as scopes were not fully pushed when the solver gets the callbacks for new equalities and new disequalities.
It also adds equality and disequality interfaces to the sat/smt solver version (which isn't being exercised in earnest yet)
