Improve Performance of source_info.mk()

[Jrendl]

Inspect.stack is a resource intensive call as it recursively gets all the context for every stack frame (see https://stackoverflow.com/questions/17407119/python-inspect-stack-is-slow). By only calling it once, I was able to cut the time spent in calls to the vsc.constraint decorator in half.

[mballance]

Excellent observation, @Jrendl! Changes all look good and tests pass