Support countably infinite regular expressions

[k15z]

The goal is to support array-based indexing for regular expressions with unbounded repeats. Currently, the max_repeats parameter limits the number of times any sequence can be repeated, making it so that there are always a finite number of strings which can be generated from a regex.

After this change, the user will be able to choose between (1) specifying max_repeats and having a finite set of strings or (2) not specifying max_repeats and having an infinite set of strings they can iterate over and/or index into.

Repeat

Modify the Repeat class to apply Cantor's pairing function when max_repeat is not specified.

x -> (a, b) # decompose the index into two values
b -> interpret this as the length of repeated sequence
a -> (a1, a2, a3, ... a_b) # convert it into `b` values
a_i -> the integer index of the `i`th element in the sequence

Note that the length attribute will be set to float(-inf).

Choice

Modify the Choice class to handle both finite nodes and infinite nodes. It should assign the smallest integers to the finite nodes; then, once those are all assigned, it should start handling the infinite nodes by rotating between them.