V4 debugger: change agent API to work on all debuggers.

From a client point of view, there is no way to know which engine is
executing which script. So, instead of passing in a debugger for a
specific engine, have the agent set breakpoints on all debuggers.

Change-Id: I53b8cbc1bdc02c7705042f2ac3a99a041992c134
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
V4: Constant binary expression evaluation.

Statically calculate binary expressions which have two constants as
operands. Currently only for add/subtract/multiply/divide and
comparisons.

Change-Id: Ia8c7222d45bbba956025fe349fc1494015a3e74f
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>