#17 Record updates of polymorphic records

[Michael Norrish]

It might make sense to make record update functions
more polymorphic, allowing the types of polymorphic
fields to change. If rcd is a type with a fld1 that
includes a polymorphic type (over :'a say), then define
fld1_fupd to have type

:('a fld1 -> 'b fld1) -> ('a rcd -> 'b rcd)