Add %ffi as variation of %raw with arity checking.

[cristianoc]

Add %ffi extension which is a variation of %raw taking arity into account.
The argument string is parsed as JS function, the arity extracted, and used to wrap the expression in a type constraint.
For example, arity 2 constrains to type (_, _) => _ so just the shape of a function with 2 arguments.

See #6213

% cat tst.res           
let canUseCanvas: unit => bool = %ffi(`
  function canUseCanvas() {
    return !!document.createElement('canvas').getContext;
  }
`)

[zth]

👍 Great stuff.