EC: decode_bool() simplifications

Ronald S. Bultjerequested to merge
rbultje/dav1d:unlord-ec-merges into master

Merge two patches from !423 (closed):

  • introduce decode_bool_equi(), which decodes a bool with a fixed 1/2 probability;
  • move EC_PROB_SHIFT to be a private define inside msac.c rather than part of the public EC interface.

CC @unlord and @atomnuker.

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