free variables in-line python

[yaythomas]

For both !py -strings and pypyr.steps.py, add support for free variables.

This is less edge case than it might sound: it comes up with very unfriendly and not easily understood errors when you attempt to use nested comprehensions:

[(x, y) for x in list1 for y in list2]. In this case, list2 will not resolve even if it is in context, leading to much WTF-ing for something that looks like it should work and in fact does work when you run it against the normal python compiler.

These pypyr inline python functions rely on eval and exec under the hood. Both of these, although taking both a globals and locals argument, only resolve free variables against globals.

[yaythomas]

See adr 0001-namespace-on-eval-and-exec