Support {} for empty record literals and types.

[cristianoc]

Example:

type allOptRec = {n?: int, s?:string}

let construct = (b) => b ? {n:0} : {}

// let z = {}
// Error: Empty record literal {} should be type annotated or used in a record context.

type emptyrec = {}

let er : emptyrec = {}

[cristianoc]

Uses: rescript-lang/syntax#632

[cristianoc]

Note: only handles expressions so far, not patterns.
Not clear there is missing expressivity with patterns. As long as one optional field exists, one can match on it being None.

No need for {} in patterns as it's the same as _.