ec_smallmul: Convert CDFs to iCDFs.

Hoists the iCDF conversion outside of the daala code.
We directly store 32768 - cdf[i] in each cdf, to avoid having to
convert the whole array every time a symbol is coded.

This works with ec_multisymbol, new_tokenset, and ec_adapt.

Compared to Change-Id Idbbd3743e9189146cb519d5b984bdabd69e3f4c0,
this improves decoder runtimes by 1.15% at QP=55 and 2.64% at
QP=20.

The overall slowdown of ec_smallmul is now 0.12% at QP=55 and
0.44% at QP=20.

Encoder output should not change, and all streams should remain
decodable without decoder changes.

Change-Id: I06b8b75b667bb1bc4ddffc78f895e48a09f4c578