pyconvert design

- switch to using explicit branching logic, instead of lookups of functions?
- prefer copying over wrapping? e.g. `list` to `Vector` not `PyVector`?
- arguments for finer control? e.g. `copy=true` to prefer copying over wrapping

related: #355

Start by reviewing the pyconvert implementation and the related issue #355. Resolve the design questions about explicit branching versus function lookups, copying versus wrapping, and optional control arguments; done means the project has an agreed direction and defined behavior for these cases.

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