This directory is workaround to avoid build error of GYP.

parser.y was located at rewriter/calculator/, but it
caused build error of calclator.gyp:calculator.  GNU make tried to call yacc
to process parser.y, though there was no rule about parser.y in gyp files.

This error can be avoided by moving parser.y to some directory other than that
of parser.c, so I moved it to this directory.