Make infinity rings a subclass of singleton

[tscrim]

Subclasses of Singleton are over 3x faster when recreating the ring:

sage: from sage.misc.fast_methods import Singleton
sage: from sage.rings.infinity import _uniq
sage: class Foo(_uniq, object):
....:     def __init__(self):
....:         pass
sage: class Bar(Singleton, object):
....:     def __init__(self):
....:         pass    
sage: %timeit Foo()
The slowest run took 71.72 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 306 ns per loop
sage: %timeit Bar()
The slowest run took 400.59 times longer than the fastest. This could mean that an intermediate result is being cached.
10000000 loops, best of 3: 94.6 ns per loop

Also IMO, it is a more Sage standard class than the _uniq, and so, more maintainable.

CC: @rwst @paulmasson @eviatarbach

Component: symbolics

Author: Travis Scrimshaw

Branch/Commit: 099d7ce

Reviewer: Ralf Stephan

Issue created by migration from https://trac.sagemath.org/ticket/20831

[tscrim]

Branch: public/rings/make_infinity_ring_singleton-20831

[tscrim]

Commit: 099d7ce

[tscrim]

New commits:

099d7ce

Make the infinity rings subclasses of Singleton.

[rwst]

comment:2

Passes tests with patchbot and LGTM.

[rwst]

Reviewer: Ralf Stephan

[vbraun]

Changed branch from public/rings/make_infinity_ring_singleton-20831 to 099d7ce