Add option for different kinds of storage in .stored_nsvars and .retained_tempvars

[bskinn]

Currently it's just an assignment, which means that the contents of any referenced mutables stored in .stored_nsvars will update if those variables are later updated/appended-to.

At a high level, it might be useful to allow callers to specify how they want variables stored in .stored_nsvars among 'assign', 'copy', and 'deepcopy'.

It might also be useful to allow the same selection for .retained_tempvars, since if a mutable is assigned to both a masked and a non-masked variable, a modification to either the surviving non-masked variable or the .retained_tempvars variable will also be reflected in the other variable.

Important: For the most natural behavior, any variables restored to the namespace at __exit__ should probably be the original variables. Thus, if .copy or .deepcopy is indicated, a separate version of the stored nsvars that's constructed using simple assignments should probably be curated.