Carried over previous changes in src/erlang_fast_segment.erl to src/erlang_fast_field_encode.erl

**the previous commit broke erlang_fast!**

More specifically, changes in the return values of `erlang_fast_segment:encode/3` were not carried over to `src/erlang_fast_field_encode.erl`, breaking the `encode` function. The main reason of the pull request is to rectify this.

This pull request also uncomment and update the previously commented out tests in `src/erlang_fast_test.erl`.