Adds a CELT_GET_LOOKAHEAD() ctl() call instead. Other uses of
celt_mode_info() should not be needed anymore.

Got authorization from all copyright holders

by POSIX. The other _t types that are not part of the API are still there
for now. Also, got rid of all that was left of the 64-bit types.

creating the mode. This means that the same mode can be shared for mono and
stereo.

Add some paranoid validation to the header functions.

that the passed size parameter is not < 56.

use it now.

to multiple MDCTs per frame. Not complete yet.

mono