v4 syntax TODO

[jordwalke]

Here is the current stack of diffs for v4 features.

This task list tracks the remaining items that make sense to do in a breaking release.

• Support for reason version tagging at start of file [@reason.version 3.7];
• String template support using backticks like ES6, and polymorphic variants now use tag syntax #LikeThis.
• String template literals normalize leading whitespace unlike ES6.
• Type parameters use angle brackets type doubleList<'t> = list<('t, 't)>.
• Automatically update entire reason file to v4 editor if detecting use of angle brackets for type parameters.
• Fix parsing precedence of string concatenation ++ and various operators to be left biased in the AST so that x ++ y ++ z is (x++y) ++ z.
• Figure out how to interpolate printing of types in string literals. Should we use the same construct as printf for this?

        let x = `
          By default a string is expected here ${someVariableThatIsAString}
          But what about an integer %d{int here}
          And what about a custom datatype %%{myCustomDataType, printer}
        `

• Decide identifier renaming (or not) scheme once and for all to avoid all identifier conflicts/ambiguity with ocaml identifiers.
• { } should not be an empty object and not be the empty object type. Should possibly be unit in expression and something else in types. Empty objects are seldom used.