Add oc_enc_tell to report an estimate of the number of bits used so far.

Remove the special case for 0 in EC_ILOG, as we never pass it 0 and this
 generates slightly better code.

Update ec_enc_bits64 to split the encoded values along word boundaries...
 this should generate slightly better code, as well as fix a subtle bug (the
 proper bits were not being masked out of the low part).
However, this will render previous streams that used this function undecodable
 (to my knowledge, no one is actually using it yet).

git-svn-id: http://svn.xiph.org/trunk/ghost@14391 0101bb08-14d6-0310-b084-bc0e0c8e3800