cleanup variable factories

[videlec]

Various parents need to create variable names (the polynomial rings, free algebras, groups, the symbolic ring, etc). We fusion all of them.

1. the following will now raise TypeError

sage: R.<a,b> = PolynomialRing(QQ, ('a', 'b'))
sage: A.<x,y,z> = FreeAlgebra(QQ, 'x,y,z')

(see the "polynomial ring" thread)

2. The many versions are normalize_names are fused in sage.misc.variables.normalize_names

3. Deprecations:

   • the var_array argument in PolynomialRing
   • the methods normalize_names, _certify_name of CategoryObject

4. Problems with injection of reserved names

sage: a = SR.var('or')   # this should work
Traceback (most recent call last):
...
ValueError: The name "or" is not a valid Python identifier.
sage: QQ['or'].inject_variables()   # this should not
Defining or

note: should take care of '_', 'True', 'False', 'None' and builtins (see the "reserved name for variables" thread)

As a sided effect, we got some speedup in polynomial ring creation!

See also:

• "polynomial ring (bis)" thread about the allowed form for creation of PolynomialRing, FreeAlgebra, etc

CC: @mforets

Component: algebra

Author: Vincent Delecroix

Issue created by migration from https://trac.sagemath.org/ticket/18390