Cherry-pick Daala 211c2a41: Clean up EC tell() and tell_frac() functions.
Add a const qualifier to the od_ec_enc and od_ec_dec parameters of
 the od_ec_enc_tell(), od_ec_enc_tell_frac(), od_ec_dec_tell(), and
 od_ec_dec_tell_frac() functions.
Add an OD_WARN_UNUSED_RESULT to od_ec_enc_tell_frac().

Change-Id: Ia50e2fd75e98d8a03d993449d658b695cf56e6fb