wscanf and vwscanf implementation proposal

Following the work done in !472 (merged) , here's an implementation proposal for wscanf and vwscanf (the latter does not appear in this list).

The implementation of wscanf.rs remains unchanged, I've just added a way to process a file in the LookAheadReader (the same way it is done in scanf) but with the ability of processing multi-byte characters. The logic to recognize the encoded length was taken from the fgetwc implementation.

I did not find tests for the scanf function (vsscanf and sscanf are the ones tested) and I could not figure how to test the new implementations. I tested them manually and they seem to work fine.