Make SL2Z distinct

[craigcitro]

This patch changes SL2Z to be a distinct object, as opposed to a class. The following error was brought up on sage-support:

sage: S = SL2Z()([0,-1,1,0])
sage: T = SL2Z()([1,1,0,1])
sage: S*T
...
<type 'exceptions.RuntimeError'>: There is a bug in the coercion code in SAGE.

The issue (as the poster pointed out) is that the parents of S and T are distinct copies of SL2Z, when they don't need to be. Indeed, I don't see any difference between this and other distinct rings in Sage (such as ZZ, QQ, etc), so I've made it distinct.

Now the above becomes:

sage: S = SL2Z.([0,-1,1,0])
sage: T = SL2Z.([1,1,0,1])
sage: S*T
[ 0 -1]
[ 1  1]

Component: modular forms

Issue created by migration from https://trac.sagemath.org/ticket/3275

[craigcitro]

Attachment: trac-3275.patch.gz

[rlmill]

comment:1

looks good to me, but i haven't tried applying the patch or testing...

[sagetrac-mabshoff]

comment:2

Merged in Sage 3.0.2.rc0. Testall long passes.