16-bit int fixes.

This fixes a number of issues for platforms with a 16-bit int, but
 by no means all of them.
The type change for ec_window (for platforms where sizeof(size_t)==2)
 will break ABI (but not API) compatibility with libsilk and libopus,
 and reduce speed on x86-64, but allows the code to work in real-mode
 DOS without using the huge memory model, which is useful for testing
 16-bit int compliance.