This adds support for parsing a String into a Float and an Integer.
There are two ways of doing so:

1. By sending `to_integer` or `to_float` to a `String`.
2. Using `Integer.from_string` or `Float.from_string`.

Using `to_integer` and `to_float` will perform a lossy conversion:
returning 0 or 0.0 for invalid input. Using the `from_string` methods
will result in a strict conversion, with an error being thrown for
invalid input.

Fixes #134
Fixes #156