YANG parser is indicating wrong start of statement

Our current parser grammar parses YANG files as if they are statements. This is true in general, but files can have leading and trailing whitespace, which is not part of the statement itself.

We therefore have a bit of a mess in what constitutes a statement – i.e. we allow it to start with whitespace. This leads to source references to point to thee start of whitespace rather that to the start of the keyword for a particular statement.