Minor fixes for string interpolation, where, and protocols

[jtbandes]

Addresses the following issues:

import Foo   // trailing whitespace ok

func f<T: P3>(_: T) where T.A == Self.A // trailing comment should still allow 'where' to end
func thisIsAFunctionDecl() {}

protocol P { init() }
func notPartOfTheProtocol() { } // so a body should be allowed

print("nested: \(1+"string"+2)")
// this should not still be part of the string

(To solve the last one, I resurrected the begin/end patterns we previously had for string interpolation. Resolves #25. cc @revolter)

[revolter]

Off-topic question, why is @infininight assigned if you, @jtbandes, made the commits and the merge too?

[jtbandes]

I was hoping to get his feedback, but I decided it was worth merging; we can always make changes later if desired.

[revolter]

Thanks for the explanation. Thought it was some management stuff I didn't know about and could learn from.