Refactor SR through a wrapper class for symbolic equations, inequalities

[mkoeppe]

We create a "subring" of the Symbolic "Ring" that only includes value objects without variables (like SymbolicConstantsRing) and has eager evaluation of relational operators.

Compare:

sage: s = SR('x') == 1                                                                                                                                                           
sage: s                                                                                                                                                                          
x == 1
sage: s in SR                                                                                                                                                                    
True

with:

sage: SymbolicValuesRing(SR('x'))
x
sage: SymbolicValuesRing(SR('x') >= 1)
error - cannot convert
sage: SymbolicValuesRing('x') >= 1
error - cannot decide
sage: assume(x >= 2)
sage: SymbolicValuesRing('x') >= 1
True

This could then be used for example with Polyhedron(..., backend='field').

Approach 1 to implement this is to wrap elements of SR in some wrapper class.

Approach 2 is to implement it through refactoring:

• We refactor the Symbolic Ring so that there is a convenient parent class for symbolic expressions with normal Python comparison semantics instead of creating relations.

• The magic behavior of comparison operators for SR elements, creating relations, will be implemented in a wrapper class.
  Ideally, this will be unified with the separate implementation of this functionality in sage.numerical.linear_functions.

Here we do Approach 1.

CC: @mjungmath @egourgoulhon @DaveWitteMorris @yuan-zhou

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/30234