WIP: Cythonize away some perf hot spots

[leofang]

Description

Less aggressive version of #677. PR description to follow later. Still WIP.

Preliminary data:

• cuda.core main branch

In [5]: %timeit e = dev.create_event()
4.65 μs ± 18.4 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

• this PR

In [6]: %timeit e = dev.create_event()
1.11 μs ± 6.91 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)

• cupy

In [8]: %timeit e = cp.cuda.Event(disable_timing=True)
749 ns ± 5.45 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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