Add a function which turns the current GAP "backtrace" / execution context into a Julia Base.StackFrame object

[fingolfin]

to be called from error handlers. Work towards issue #45.

A starting point is the GAP function Where resp. WHERE.

Note that there are some existing issues with that, which are not necessary to be resolved for implementing this, but which affect the quality of the stacktrace; see

• Error backtrace should print names of operations/attributes/... in which the error occurred gap-system/gap#1196
• Bug in interaction of DownEnv, UpEnv, and Where gap-system/gap#4577
• ...