Fix brittle function export

[okennedy]

Describe the bug The current function export works with spit and duct tape, and there are multiple ways to break it. For example:

To Reproduce

def foo():
  print("hello")
bar = foo
vizierdb["x"] = bar

You will get a 50/50% chance of the export failing.

Alternatively, attempt to export a function imported from another file.

Expected behavior

Currently, Vizier uses a bit of a nasty hack to find the source code of the variable being assigned from. However, the __code__ field provides much of this metadata already. Notably:

• __code__.co_name is the name under which the function is defined (this fixes the 50/50 failure chance named above)
• __code__.co_filename is the file that provided the function (this should address the import issue)

[okennedy]

This should also open up the possibility of extracting symbols from different files more easily.