Type deduction for records

[julianhyde]

Improve type deduction for records. For example,

fun hasJob e job =
  e.job = job;

throws

java.lang.ClassCastException: net.hydromatic.morel.type.TypeVar cannot be cast to net.hydromatic.morel.type.RecordLikeType

and has to be changed to

fun hasJob (e: {name: string, job: string, empno: int}) job =
  e.job = job;

in order to compile. Ideally we would infer record types (or constraints on record types) but at a minimum we should give a good error, not throw ClassCastException.