wire_ctx: refactor to reduce duplication and UB

It was using misaligned pointers which causes undefined behavior, at least theoretically.

API change: data in wire_ctx_read() is now passed as void* instead of uint8_t*, and it zeroes the data on error.

Standard diffs aren't well readable, as I had to move a few of the functions.

It is all okay according to me.

mentioned in commit a1874713

merged