Support casting from string in embed

[feymartynov]

I use PostgresSQL's JSON_AGG function which serializes rows from a joined table into single JSON field and then use Ecto's embeds_many to inflate them into structs. These structs have Arc type field. When Ecto calls cast/2 with a string of the form file_name?updated_at as the second argument it matches the last definition which tries to store this string and fails but I expect it to do the same things as load/2 does in this case. So here is my patch for it.

[vyorkin]

hey guys! is it going anywhere? cc @stavro