allow mutation of private record fields with @allowMutation

[tsnobip]

this PR allows this:

module Foo: {
  @allowMutation
  type t = private {
    mutable foo: int,
  }
  let make: int => t
} = {
  type t = {
    mutable foo: int,
  }
  let make = foo => {foo: foo}
}

let foo = Foo.make(10)

foo.foo = 5

Using private for records is very convenient to bind to JS classes so you can easily access properties and methods while preventing object creation. Though, before this PR, the mutation of fields marked as mutable inside private records was not possible and that is a common limitation inside WebAPI bindings, for DOM.node for example where textContent should be mutable.

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commit: 12f8390