To do: in-place API

Make in-place versions of all the API functions, like:
```julia
pyimport!(ans, "sys")
pyadd!(ans, x, y)
pygetattr!(ans, x, "foo")
```
These replace the pointer in `ans::Py` with the result of the computation, instead of allocating a new `Py` each time. This will allow writing super-fast non-allocating code where needed.

You'd use these by pre-allocating places to store the results with `pynew`, which can optionally by freed with `pydel!`.