Add replace function

[pineapplemachine]

Should follow #58

I have some ideas about how to maybe generalize this kind of find behavior with FSMs

You could pass a finder a consumer

A consumer is an object with consume and state and copy methods. consume accepts a sequence element as an argument and advances the consumer by one element. state returns one of active, completed, terminated. Active is the in-progress state, completed is the successfully-finished state, and terminated is the failed-finished state. copy forks and makes a copy of the consumer. A consumer could be paired with a finder to find substrings matching some other sequence or to find substrings of consecutive elements matching a predicate, or could be more complicated and do things like detect floating point numbers. A long term goal might be to build these lazy consumer objects from regular expressions

The split function would change to accept a consumer, or a sequence or a predicate to construct a simple consumer from. Same with replace when added.

[pineapplemachine]

Depends on #99