The methods Sequence.map and Sequence.where are now implemented using
deferred execution. They return an instance of a new Sequence-derived
class that performs the operation while iterating. This has three main
advantages:
* It can be computationally cheaper when not the whole sequence is
iterated.
* It consumes less memory since it does not store the result in a newly
allocated list.
* They can work on infinite sequences.
Some disadvantages are:
* Iterating the returned iterator will be slightly slower due to
the added indirection.
* You should be aware that modifications made to the original sequence
will affect the returned sequence.
* If you need the result in a list, you now need to call Sequence.list
on the result.
When possible, they return the actual value from the predicate
instead of always just "true" and "false". This matches && and ||
which evaluate to the RHS or LHS when appropriate.
