require all memory reads to be aligned.  Specifically, it turns out that
sizeof(RepeatedField<bool>) is 20 on 64-bit sparc with GCC 3.4.6.  This is
strange, since one of RepeatedField's members is a pointer, which I thought
meant that it had to be 64-bit aligned, which means its size should be a
multiple of 64 bits.  But, 20 is not a multiple of 8.  I don't understand why
this is the case, but if this is possible, then DynamicMessage's strategy of
sorting fields in descending order by size and then tightly packing doesn't
work.  To fix this, I got rid of the sort step and instead added code that
aligns each field's offset appropriately based on the field's size.

Also in this revision:  Fix an error message that named a flag incorrectly.