Laurel: field access on Any-typed values

[julesmt]

Adds the read-side bridge that lets Laurel resolve and read a field on a value
whose static type is Any (a class instance carried by reference), by unboxing
it to its heap Composite. Gated on the program defining an Any datatype, so
existing Laurel programs are unaffected.

Example:
Python values often reach Laurel typed Any (unannotated parameters, spec
self, heterogeneous containers). Reading an attribute then has to recover the
underlying object:

class Account:
    balance: int

def is_rich(a):                # `a` is unannotated → `Any` in Laurel
    return a.balance > 1000    # attribute read on an `Any`-typed value