Buffer: Move toVarMap()
Combinators: Fix signature of partialMap()
VarBuf: Implement trait MapFunctions
OptVar: Should be a channel of Option[Var[T]]
Combinators: Split FilterFunctions

FilterFunctions should only contain combinators operating on values.
ReadVarBuf: Add toVarBuf()
Combinators: Fix signature of flatMap()
Combinators: Add splitAt()
ReadChannel: Add flatMapCh()
VarBuf: Rename unit() and overload initialiser

It can be used for ReadBuffer[Var[T]] as well.
OptVar: Change semantics for better usability
Combinators: Add equal() and clarify contains()
Combinators: Clarify head() and derivatives
Combinators: Add unequal()
VarOpt: get() should dereference the value
OptVar: Rename to Opt