Add row polymorphism macros to Michelson

With Babylon, we have entrypoints: the contract parameter is a tagged union of types, where each element represents an individual entrypoint.

This idea could be extended to product types:

• Construction behaves the same
• A destruction macro is added, equivalent to some combination of CAR's and CDR's:

:: 'a : 'A -> 'b : 'A
   iff field_type 'label 'a :: 'b
GET_FIELD 'label / a : S => b : S

• An update macro is added:

:: 'b : 'a : 'A -> 'b : 'A
   iff field_type 'label 'a :: 'b
SET_FIELD 'label / b : a : S => b : S

• DIP_FIELD?
• contract t then would represent any contract with an entrypoint t with at most the given product elements.
  • E.g. contract (pair (nat %counter) (bool %verbose)) may refer to a contract with parameter type (nat %counter)

What are the benefits?

• Code/lambdas accessing product elements may be composed without manually adapting them to different products
• Code to access some deeply-nested element of a product may be determined from the choice of product element
• Entrypoints become more flexible, allowing callers to provide at least the given arguments
• Callbacks become more flexible: e.g. if a contract accepts a callback of parameter type (pair (nat %account_balance) (bool %is_admin)), we can provide a callback of type (nat %account_balance) and ignore the (bool %is_admin) field
• High-level code that compiles to Michelson could benefit
• In theory, GET_FIELD may be compiled to accessing a struct of all fields, which is more efficient than traversing a deeply-nested composition of pairs.