basic assumptions on symbolic functions

[rwst]

At the moment only symbols allow basic assumptions (i.e. generic declarations). Newly defined functions are ignored in GDecl and then bail out (because Expression.assume only can work with relations). Preinstalled functions will have statements about them in the assumption database, as well, and maybe it's useful for the user to be able to make assertions, but Function is ignored as well and bails out for the same reason as above.

The Pynac side will have a first implementation in 0.4.3.

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/18999