Refinement code requires private types to reside in
separate program declarations. So we split type decls
into chunks where all non-free types are declared
separately and only constructible (Ddata) types are
kept together. The code preserves the original order
wherever possible.

Also, export ls_of_rs and fd_of_rs from Expr: these are
used everywhere in src/mlw anyway.

Also, remove some range/float-related "assert false".