Derivation implemented for function calls (only). Eg. f'(2). It is not
yet possible to do something like f''(2), but this should be implemented
in the future. It should also be possible to derive normal expressions,
but this is not yet possible.
Previously, eg. "dx" would not be parsed as "dx" after a function, since
the parser did not keep track of whether or not it was currently inside
an integral or not, properly. This commit fixes that, and also makes it
possible to use any variable after the "d", eg. "dy". The integration
function was also put in its own file: calculus.rs.
