Add `NamedSequence`, `NamedBlock` wrapper classes

[LTLA]

These basically mimic R's names for lists, NumPy arrays, etc. with as little effort as possible, rather than converting them to dictionaries all the time.

[LTLA]

Actually, what I want to do is more easily achieved by just creating IntegerList, FloatList, and BooleanList subclasses from NamedList, much like for the StringList class. These would be analogous to R's atomic vectors.