Moved selection parser helpers into separate file.

Moved selection parser helpers into separate file.

Makes parser.y much more streamlined, as there is now nearly only Bison
declarations and the actual semantic actions.  Also, editing the helpers
no longer requires regenerating the parser (which most likely also
changes the line numbers in parser.cpp, which is typically the largest
part of such changes).

Change-Id: I60e76b95725fb9f647f3cddf64773b3f94185b06