Add ring and promote methods for Constant objects

[d-torrance]

Add support for creating matrices with Constant entries. Also add a unit test.

Before

i1 : matrix {{ii}}
stdio:1:6:(3): error: common ring not found

After

i1 : matrix {{ii}}

o1 = | ii |

                1         1
o1 : Matrix CC    <-- CC
              53        53

Closes: #3456

[pzinn]

looking good. it still misses some cases, though maybe we're reaching the limit of the promote system, cf #3184:

i1 : f=map(CC[x],CC[y,z],{x,ii})
stdio:1:5:(3): error: no method found for applying promote to:
     argument 1 :  ii (of class Constant)
     argument 2 :  CC  [x]
                     53

[d-torrance]

That case was straightforward enough. I added promote(Constant, RingElement), so now your example works:

i1 : map(CC[x],CC[y,z],{x,ii})

o1 = map (CC  [x], CC  [y..z], {x, ii})
            53       53

o1 : RingMap CC  [x] <-- CC  [y..z]
               53          53