utils/bits.h: Refactor the load/store macros.
* Each macro is now only defined conditionally. This allows earlier
parts of the header file to define a compiler-specific
implementation, with the generic definitions filling in the blanks.
* Instead of writing the byte manipulations out longhand, build each
one out of smaller pieces. This has two benefits: firstly, the new
definitions are smaller and easier to understand; and, secondly, if
there's a compiler-specific trick for, say 16-bit loads, then 32-bit
loads can win some of the benefit.
~mdw / mLib / commit