Add 'hiding' to reservedMeta in Haskell.bnf and regenerate files.

Imports hiding things are currently not highlighted in the 'hiding' part of the import statement. The keyword exists in a later chapter in the standard:

http://www.haskell.org/onlinereport/haskell2010/haskellch10.html#x17-17700010.2

How do you feel about adding support for parsing GHC extensions in general?