Use boolcoder API instead of inlining

This patch changes the token packing to call the bool encoder API rather
than inlining it into the token packing function, and similarly removes
a special get_signed case from the detokenizer. This allows easier
experimentation with changing the bool coder as a whole.

Change-Id: I52c3625bbe4960b68cfb873b0e39ade0c82f9e91