Replace explicit `get` on computed properties

See github/swift-style-guide#1.
s/observable/signal/
Allow Actions to be initialized with a Scheduler

An action will deliver results upon and synchronize itself with this
scheduler, allowing UI or background actions to be defined.

@robrix I decided against making this a type parameter, since the only
useful distinction (IMO) is background vs. not background, and that’s
not really captured in the Scheduler type family currently.
Remove unnecessary Foundation imports
Rename Bag.add -> insert
Make Consumer final
Rename _dummy… methods to _empty…
Remove unnecessary temporaries in scheduler bridge
Add & use case analysis over Optional type
Infer RACEventType
Clarify Producer, and its relationship to Signal
Remove confusing “stream-of-streams” language
Clarify what `nil` means for mapAccumulate