Roles

Split effective and granted access.
Add "role exists", "role doesn't exist" errors.
Test role create and drop.

Rename space_check_access() to access_check_space()
to follow the convention.
box.session.su('role') is not allowed

Implement box.schema.user.info() which print user's grants
Merge branch 'roles'

Conflicts:
	src/box/lua/schema.lua