Initial support for VDB python-based variable objects
Due to a limitation of GDB itself, one cannot create variable
object for "synthetic" argument/local created by custom frame
decorator. Therefore one cannot "dig deeper" and inspect contents
of such a variable in case it is a kind of composite - like an
object on a heap referring to another objects.
This is a problem when a debugee is does not have debug information
and / or it's language is not C - such as when debugging JITed code.
This (experimental) commit adds a parallel implementation of variable
objects done completely in python. This new varobj MI interface is a
super-set of GDB's builtin varobj MI interface.
