Introduce 'public' role.
Auto-grant 'public' to every user.
Update the snapshot.

On ARM chars are unsigned

This code didn't worked on ARM, changing the table and parameters to int seems to work fine on any platform.

Track granted roles.
Represent roles granted to a user with a bitmap.
Disallow loops in role grants.
Add tests for box.schema.user.grant('username', 'rolename')
Add basic tests for loops in grant sequences.

Rewrite largely buggy code in authentication token lookup.
Do not waste space on authentication token bitmap, correctly
check for overflow of the bitmap.

#638, checking/configuring in cmake pthread_yield and sched_yield; READLINE_ROOT for better readline control;

A few renames.

Rewritten

Write a function which can be used to check whether a user
exists or not.

Fixes gh-632.