Consistency and redundancy of verbal assumptions

[rwst]

Maxima's assumption framework (still used by Sage's GenericDeclaration) has only rudimentary consistency and no redundancy checking.

sage: assume(x>0)
sage: assume(x,'real')
sage: assume(x,'imaginary')
sage: assume(x,'complex')
sage: assumptions()
[x > 0, x is real, x is imaginary, x is complex]

This ticket should wrestle the GenericDeclaration consistency checking (not the same with relations!) from Maxima, implementing it directly in Python, only informing Maxima about any such assumption. And by the way add the positive and negative keywords.

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/19670