Add `assemble` methods in Courier record companions that use field inclusion

[josh-newman]

Right now, records that use field inclusion have the standard two construction methods in their companion (building from DataMap and with all fields specified explicitly). I think it'd be useful to add additional methods, perhaps called assemble, that accept the included arguments as records instead of individual fields. For example:

record A {
  field1: int
  // many others
}

record B {
  ...A
  fieldB: string
}

I'd like to construct B with B.assemble(A(...), "fieldB"), instead of B(1, ..., "fieldB").

For records with multiple inclusions, I think generating a single assemble that accepts all included records plus all additional fields is sufficiently useful (rather than worrying about the complexity of overloads for different inclusion combinations).

[marcgrr]

+1

Also, some opposite disassemble (or deconstruct or unapply) methods would be useful and they would have prevented the example bug that I describe in #60 .