Remove strict aliasing violations from our C code

[clyring]

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.