The Return to Compilability

Updated for Xcode 6 beta 3.
Get rid of some uses of Box
Signal.{takeWhile,filter} should return optionals

Then, no initialValue is required. If you want a non-optional signal,
simply chain through ignoreNil.
ignoreNil -> unwrapOptionals, add forced variant
Refactor Signal to always require an initial value

Don’t allow a backdoor in the constructor itself.
`forceUnwrapOptionals` can be used instead, for cases where you _really
know_ that there will be an initial value, and just can’t easily prove
it.