Propagate expected dict value type into dict literal typing

[zth]

Currently, this fails:

module Hidden = {
  type routeHandlerObject = {get: int}
}

let dictValueInference: Dict.t<Hidden.routeHandlerObject> = dict{
  "health": {get: 200},
}

let primitiveMakeValueInference: Dict.t<Hidden.routeHandlerObject> = dict{"health": {get: 200}}

It fails because dict{} expression is emitted as Primitive_dict.make(["health", {get: 200}]), and that makes it loose type info. This PR propagates the expected dict type info further, which means the above code will work.

As a nice side effect, this PR also improves type error messages for dict. This:

let x: dict<JSON.t> = dict{"1": 1.}

Will currently fail on the whole dict{} (although it's just 1. that's wrong). After this fix it'll fail on 1. instead (Expected JSON.t, got float).

@cristianoc can you think of a better cleaner way to do this?

Also, Codex is saying these cases are also broken and could be fixed the same way:

1. async
2. spread arrays/lists
3. tagged templates

These fail today:

module M = { type t = {get: int} }

// Fails
let f: unit => promise<M.t> = async () => {get: 1}
// Works (no async)
let f: unit => M.t = () => {get: 1}

// Fails
let xs: array<M.t> = [...[], {get: 1}]
let xs: list<M.t> = list{...list{}, {get: 1}}

// Fails
let tag = (_strings, values) => values[0]->Option.getOrThrow
let x: M.t = tag`a${{get: 1}}`

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commit: 19b5ddd

[cristianoc]

Legit.
I guess a first class support for dict would avoid the issue, but might be more invasive.
(And async etc).