model: fix o:set()
object:Table: add .available() method

to get an array of free IDs
sancus.model: don't add context to validation errors
Table: pcall() o.init on new()
Table: add :after() and before() helpers
Table: fix lower limit in table iterator
utils: add assertish() - just like assert but not prefixing errors
utils: add keys() to iterate over sorted keys