utils/bits.h: Support compiler magic for unaligned loads and stores.

Introduce `RAWw' macros to access the raw memory, and implement `LOADwe'
and `STOREwe' in terms of these and `eTOHw'/`HTOew'.

These are remarkably tricky because GCC (I think mistakenly) thinks that
type-based aliasing is applicable when in fact the other type in
question is `unsigned char', which is known to be able to alias
anything.  Hit things with the `may_alias' hammer and hope they quieten
down.