Remove dependency on postcss-functions

[adamwathan]

This PR removes our dependency on postcss-functions by implementing the logic we need ourselves. The primary motivation for this is to be able to throw errors from the relevant node, instead of having to throw them from the root. Using postcss-functions we had no way to access the actual node where the function was being called to throw a helpful error, but by implementing it ourselves we can make sure to keep a reference to the node and make it available to the actual function.