Implements functions

[nicklayb]

We would like a way to support common functions and user defined one. It might be useful to be able to use now() to refer to the current timestamp for instance or a way to get a contextual record.

Requirements

Support an identifier followed by parentheses like function("something") or now().

Goals

Being able to write get Event where start_time > now() that would convert to Database's NOW() function when querying.
Being able to write get Event where id = current_id() that would return a given value from the assign's map.

API proposal

Implement a function invoke/3 in EctoDefinition with the following signature

def invoke(function :: atom(), args :: list(String.t()), assigns :: map()) :: Ecto.Queryable.t()

Where assigns would be usual user provided assigns containing various properties (like role or permission for instance). Invoke could return a raw value or a queryable, like a fragment.