Rearrange extension module initialization.

Introduces acid_init_module(), which is like Py_InitModule except that
the result is manually inserted into sys.modules, avoiding the need to
have a separate .so for each extension namespace (thus avoiding the need
to introduce a .so for common code, or linking common code into every
.so).

Now upon 'import acid._acid', not only is that extension namespace
created, but also 'acid._keylib' and 'acid._iterators', etc. This has
the fun side-effect of centralizing the ACID_NO_SPEEDUPS env check in
__init__.py.