Refactor C Interface to hide implementation details
Fixes sorting bug with included_files status array

With string input, the "stdin" entry would get sorted away
from the start of the array. This lead to a strange bug
since we skip the first item when copying that information
back to the implementer.
Implement @warn and generic function overloading

Define a custom function named '@warn' to get called when
the corresponding statement is found in sass code. Use a
function named '*' as a fallback, if the requested function
is not found. The original function name will be the first
argument (along with the rest of the original arguments).
Implement custom import overloading on parser