[FRONTEND] clean-up backend/jit interface

[ptillet]

This is a fairly big refactor of the JIT that prepares the field for supporting named tuples (to come in a subsequent PR):

• fixes bug in tuple specialization hashes
• simplifies launcher by always taking all function arguments (even unused constexprs)
• enables un-annotated constant expressions (necessary for variadic tuples containing constexprs)
• replaces attrsdescriptor by much more minimal backend interfaces
• unifies constexprs provided by users (e.g., block sizes, none, function arguments...) and specialization (e.g., 1)

[pawelszczerbuk]

Happy New Year, Triton!