Add a lazily-instantiated value property.
Conversions use the value property.
Remove the forced property.
Remove the boxed property.
Correct the docs.
Free up the thunk early again.
Merge remote-tracking branch 'origin/master' into delay

Conflicts:
	Hammer/Derivation.swift
Use the leaf initializer.
How did I manage that?
Much cleaner derivative of alternation.
Merge pull request #11 from robrix/delay

Delay