trans_const_expr should hash-cons to avoid duplicate constants

[catamorphism]

Or just use LLVM to do this (if it has a mode that hash-conses to avoid duplicate constants). As per FIXME in trans::base::trans_const_expr.

[catamorphism]

Relatedly, trans_const_expr should also use already-defined values when constants refer to other constants, instead of re-generating the value for the other constant (also as per a FIXME, in the expr_path case)

[graydon]

The non-regeneration thing we should fix. The hash-consing is .. optimistic, and I think probably not going to win anything at this stage. We can revisit if we find cases where it matters.