Use ffi.gc to handle object lifecycle

[bbayles]

Re: this discussion, this PR re-works how the objects from the C library are managed.

Previously I was using __enter__ and __exit__ to ensure that ada_free got called. But CFFI provides ffi.gc, which can do this automatically:

ffi.gc(cdata, destructor, size=0): return a new cdata object that points to the same data. Later, when this new cdata object is garbage-collected, destructor(old_cdata_object) will be called

This allows us to avoid the overhead of __enter__ and __exit__. On a benchmark script this saved roughly 20% of execution time.