[printer] Record fields punning

[cknitt]

type a = int
type b = string

type r = { a, b }

is printed (by BS 8.1.1 bsc -fmt) to

type a = int
type b = string

type r = {a: a, b: b}

IMHO the output should use punning for the record fields (like in the input).

[bobzhang]

shall we prioritise this issue? cc @IwanKaramazow

[IwanKaramazow]

Yes, will prioritize today.

[ryyppy]

The punning of record fields was fixed in #480.
For record type definitions, we always want to use the explicit format for clarity.

Also see the #64 discussion for future reference.