Use actual code context for enumerating

[BCSharp]

Partially addresses #1718 (when the wrapping type is list).

[slozier]

Interesting... any ideas about the underlying issue? Seems like the leak comes back when we replace list with tuple. Do we need to try to eliminate all calls that use the default context?

[BCSharp]

Do we need to try to eliminate all calls that use the default context?

I think so. The underlying issue appears to be that using the default context with objects of an arbitrary Python type (like OrderedDict, not any builtin types or .NET types) pollutes the conversion call sites (and probably any call sites) with types that cease to exist after the runtime instance is shut down.

I think it may be OK to use the default context for operations on objects of guaranteed .NET types or builtins, but not when an arbitrary object may appear as argument.

There should be more places still when the leak happens. E.g. I expect a leak when an OderedDict is splatted in a call with keyword arguments (untested, just my thought based on reading the code).

[slozier]

Sorry it took a while to review, just noticed it was no longer marked as draft...

Looks good to me.