Replace ec_{enc|dec}_bit_prob() with ec_{enc|dec}_bit_logp().

All of our usage of ec_{enc|dec}_bit_prob had the probability of a
 "one" being a power of two.
This adds a new ec_{enc|dec}_bit_logp() function that takes this
 explicitly into account.
It introduces less rounding error than the bit_prob version, does not
 require 17-bit integers to be emulated by ec_{encode|decode}_bin(),
 and does not require any multiplies or divisions at all.
It is exactly equivalent to
 ec_encode_bin(enc,_val?0:(1<<_logp)-1,(1<<_logp)-(_val?1:0),1<<_logp)

The old ec_{enc|dec}_bit_prob functions are left in place for now,
 because I am not sure if SILK is still using them or not when
 combined in Opus.