Proposal: make all PyX.create functions behave like equivalent Python constructor

[gatesn]

Currently a PyTuple.create only supports converting from a Zig tuple struct.

When wrapping/unwrapping, it's the trampoline that decides which type a value should be converted into. In some cases, e.g. for call arguments, we know we want an args tuple and kwargs dict. It is then ambiguous whether an empty Zig struct should be converted to a PyTuple or PyDict.

Similarly, we would never convert anything into a PySet. And there is an ambiguity in how to handle []u8 as PyString or PyBytes.

The proposal suggests the trampoline has a default target for each type, but all types can be explicitly constructed from any compatible object using their create function (possibly rename to new?)