Add option to disable speedups even if they were built when bezier was installed

[dhermes]

No description provided.

[dhermes]

It may be easiest to just do this in src/bezier/_speedup.pyx at import time. We could check for an environment variable (e.g. just re-use BEZIER_NO_EXTENSIONS) and than raise an ImportError if it's set.

Some other implications:

• src/bezier/__config__.py (in particular handle_import_error()) would need updating
• _HAS_SPEEDUP may need to be renamed, e.g. to _USE_SPEEDUP

[dhermes]

We could make the _curve_helpers.py (and related modules) go into a hazmat subpackage and just have them be publicly available. There is prior art for this in cryptography, trio and I'm sure other popular packages.

UPDATE: Filed this as #170.

[dhermes]

Regarding the hazmat comment, #160 made this more of a possible by putting some of these helpers in modules prefixed with _py_ to signify that they are pure-Python implementations (as opposed to a mix of Python and a binary extension speedup).