(part of meta-ticket #32414)

in sage.structure.element, we add a class Expression

• the existing class sage.symbolic.expression.Expression will be its only subclass
• deprecate is_Expression, replace uses by isinstance(x, sage.structure.element.Expression)

Follow-up in #32665: Also deprecate is_SymbolicEquation, is_CallableSymbolicExpression, is_SymbolicVariable; replace uses by isinstance(x, Expression) and a method call (may need a new method is_callable)

Depends on #32599

CC: @DaveWitteMorris @tscrim @mjungmath

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: ee7640c

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/32638