Restructure a lot of VHDL generating code.

Restructure a lot of VHDL generating code.

In particular:
 * The globalNameTable was moved into Generate.
 * A large part of mkConcSm was moved into Generate.
 * A BuiltinBuilder now accepts Either CoreBndr VHDLName and
   [Either CoreExpr AST.Expr], so we can reuse them even when we no longer
   have a CoreBndrs (this does not happen yet, though).