Remove strict aliasing violations from our C code

Although we've enabled `-fno-strict-aliasing` in #582, I would on principle rather we just didn't have any strict-aliasing violations in our code. But:
* I expect the `bytestring` C code contains little (if any) of the sort of code that actually gets performance wins from `-fstrict-aliasing`
* Although the strict-aliasing-compliant idioms are not uglier than what we are currently doing instead, they are not prettier in any way except that they remove the need to remember that `-fno-strict-aliasing` has been set.

So this is an extremely-low-priority task.

Start by inspecting the bytestring C code and the -fno-strict-aliasing change referenced in issue #582. Identify every strict-aliasing violation and replace them with compliant idioms; the work is done when the C code no longer requires those violations.

Written by the indexing model from the issue text.