Shrink CDFs by merging counter to final symbol position

[barrbrain]

A similar approach is used in dav1d and this enables some improvements to the CDF log.

[coveralls]

Coverage remained the same at 82.994% when pulling 816a7c5 on barrbrain:cdf-layout into 351f7c8 on xiph:master.

[barrbrain]

There is a change to UV_INTRA_MODES that may seem a little odd. Where the original code had UV_INTRA_MODES, it ought to read INTRA_MODES + 1 because the additional element here is the CDF counter rather than the chroma-from-luma symbol. As with other edits in this patch, we are subtracting 1 now that the trailing zero is removed. Thus UV_INTRA_MODES becomes INTRA_MODES.