Changes

Use Const_immstring for all constant strings created by the compiler

Introduce Lambda.lambda_of_const

This helper function ensures that constant strings are always
represented using Const_immstring and not Const_base (Const_string _)

Introduce Lambda.constant type

This is a copy of Asttypes.constant minus the string case.
It ensures that there is a single way to represent constant strings,
and allows to remove a few pieces of duplicated code.

Remove Lambda.Const_base indirection

Changes for PR14260