[swift-evolution] [Pre-proposal] Fix function type grammar

[Anton Zhilin]

You can view proposed function type grammar here:
https://gist.github.com/Anton3/9992aa565ff9d230dd4655b1b74a3326

Notion of tuple was removed; instead, parentheses are now merely part of 
function type grammar.

An important implication is that types () -> T  and  ( () ) -> T become 
completely different: the first accepts no parameters, and the second 
accepts a single parameter of type ().

===copy of gist begins===
function-type → ( function-type-parameters opt ) throws-annotation opt -> 
type

function-type-parameters → function-type-parameter , function-type-
parameters

function-type-parameters → function-type-parameter ...opt

function-type-parameter → attributes opt inout opt type

throws-annotation → throws | rethrows