Add new list methods

[ospencer]

Let's add methods for these:

some # checks that at least one item in the list is true for a given condition
nth # gets the nth item in the list
filter
reject # opposite of filter
count # counts how many elements satisfy a given condition
hd # gets the first item in the list
tl # returns the list without the first item
flatten # takes a list of lists and combines them into a single list
insert # takes a list, item, index. inserts the item at the index-th position
rotate # takes a list, count. item at count-th position becomes the first item. should work for negative counts as well
uniq
part # takes a list and and a number. returns a tuple containing the first n items and the rest of the list

[ospencer]

Removed every since that rename has already been done.

[designsbysm]

It looks like several of these functions should be included with Array.

[ospencer]

@JacobGinsparg is currently taking this on, and I think it's almost complete?

[ospencer]

I removed sort and sortBy from this in favor of #125.

I also added a new method, part, since it seems like it'd be useful for rotate.