Do not load libpython when precompiling

[cjdoris]

I propose that when PythonCall is being precompiled, we do not load/initialise libpython. Instead, have shims for the C functions that we use which raise an error or do something else trivial when used.

This means that we do not need to resolve a CondaPkg environment when precompiling, which will make precompilation much faster, simpler and more reliable.

This is breaking because it restricts which Python operations you are allowed to perform during __init__. I suggest that we make all of our shims return errors (clearly stating that you cannot perform this Python operation during module initialisation) except for those which return Python objects, which should return PyPtr(1) (or anything non-NULL) instead.

In particular, we should allow these operations at init:

• incref, decref, initialize, finalize and other such functions we only use internally
• pyimport, pygetattr, pygetitem, pyrepr, pystr, pycall, pyhash (always return 1)
• creation of python objects from nothing, bool, string, number, etc. (Py)
• GIL handling

And we'll need to disallow the following functions, which require an actual Python interpreter to return reasonable answers:

• pylen, pyhasattr, pynext (PyIter_Next)
• pyconvert (such as PyLong_AsLongLong etc)

Concretely, we can make shim functions like this:

shim_error() =
    error("You cannot perform this Python operation during module initialisation.")

shim_PyObject_IsTrue(::PyPtr) = shim_error()
shim_PyObject_Length(::PyPtr) = shim_error()
...

shim_Py_IncRef(::PyPtr) = nothing
shim_PyImport_ImportModule(::Ptr{Cchar}) = PyPtr(1)
shim_PyObject_GetAttr(::PyPtr, ::PyPtr, ::PyPtr) = PyPtr(1)
shim_PyLong_FromLongLong(::Clonglong) = PyPtr(1)
...

And then in init_pointers set the fields of POINTERS like this:

p.PyObject_IsTrue = @cfunction(PyObject_IsTrue_shim, Cint, (PyPtr,))
p.PyImport_ImportModule = @cfunction(PyImport_ImportModule_shim, PyPtr, (Ptr{Cchar},))
...

We should also initialise all the exceptions and other object pointers to PyPtr(1).