Remove 'function' field in {SsaTree,SsaTree2,Rssa}.FunctionEntry.T.

While a FuncionEntry.t belongs to a unique Func.t function, it isn't
necessary to record that association in the FunctionEntry.T datatype.
For instance, every Statement.t, Transfer.t, and Block.t also belongs
to a unque Func.t function, but the association is not recorded in the
corresponding datatype.  There were no non-trivial uses of the
'function' field; indeed, in most instances, the 'function' field was
simply copied verbatim from one FunctionEntry.t to a new
FunctionEntry.t.
{SsaTree,SsaTree2}.Program.clearTop should clear FuncEntry.t property lists.

FuncEntry.t-s, like Func.t-s, have global scope in an SSA/SSA2
program.  Their property lists should be cleared along with the other
global-scope entities.
Rssa.Function.clear should clear FuncEntry.t property lists.

Rssa.Function.clear clears the Func.t property list and, therefore,
should also clear the FuncEntry.t property lists.
Convert RefFlatten to multi-entry.

No significant difficulties with the conversion.
Convert DeepFlatten to multi-entry.

No significant difficulties with the conversion.