Language service: open Type.id

I am using language service to "traverse" the program, and I would like very much to "remember" which types and symbols I already saw, and to cache my computation results for them. Performance boost _and_ avoiding infinite loops. The compiler very helpfully supplies `id` for `Type`, `Symbol` and `Node`, and it seems to be "really, really" unique for [symbols](https://github.com/Microsoft/TypeScript/blob/master/src/compiler/checker.ts#L16) and [nodes](https://github.com/Microsoft/TypeScript/blob/master/src/compiler/checker.ts#L10), and "relatively unique" (i.e. within a given typechecker) for [types](https://github.com/Microsoft/TypeScript/blob/master/src/compiler/checker.ts#L40). The problem is, however, that, unlike nodes and symbols, types hide their `id`s behind the ["internal" qualifier](https://github.com/Microsoft/TypeScript/blob/master/src/compiler/types.ts#L1850) and the lack of accessor function, making them not legally visible to me as a consumer of the language service. Of course, this being JavaScript, nothing prevents me from accessing them anyway, and that is what I currently do, but this is not ideal, because I am not protected from the code evolution in the future. So it seems to me that nothing stands in the way of adding a `getTypeId` method to the typechecker, unless I'm missing something very important.