red-black nodes should carry length info

[tarakc02]

i had removed these to make the dicts/sets more compact, but it would be nice to support efficiently calculating ranks/quantiles as well as iterating from them (e.g. iterate values between 25th and 75th quantile).

could this be set up to be decided at runtime? maybe e.g. by making two different nonempty types, one with length and one without, and specialize the NE constructor based on the types of left and right. updating length is just 1+length(left)+length(right)

[tarakc02]

this turned out to be a very simple fix (just had to change two lines of code), but for a small key type (Int64), the size of the whole data structure will grow to 133% of previous size, for a feature that may or may not be useful. the main immediate use for length info would be to implement amount for #11 , but maybe that can be accomplished by storing length of the entire container (e.g. as an added field in RBDict/RBSet), which doesn't cause the same overhead.