Avoid negating unsigned numbers (for MSVC)

Avoid negating unsigned numbers (for MSVC)

MSVC doesn't like when people negate unsigned numbers, and the result is
still unsigned.  By default, it's an error.  Moreover, section 6.5.6 of
the C11 standard doesn't clearly specify what -x means when x is an
unsigned integer.  It does however specify ~x.

So I replaced -x by ~x+1.  This was getting ugly, though, so I made the
ALIGN macro.  ALIGN(x, block_size) returns how many bytes we need to
reach the next block size, assuming we've already consumed x bytes.  For
instance, ALIGN(11, 8) == 5.  It uses bit twiddling trickery, so the
block size must be a power of 2.