GitLab

Combined the myriad ScannerExceptions and ParserExceptions to a single ParserException class that has a message and a line/column position in the file where the error occurred.

Moved the input stream, together with line/column info, into its own class, which allowed some other stuff just to pass the stream, and not have to be a member of Scanner.

There's a bug (and question): should we test simple keys' validity BEFORE stuff or AFTER stuff?

Refactored some common whitespace-parsing code in scanning both scalars.
Implemented the flow collection tokens.