get model id simply, and type check

[devvit]

Two changes:

• Better way to get model id.
• Ecto cannot cast integer to string automatically, throw exception while item_id
  type is :string, Ecto.UUID <=> :string is well done.

[izelnakri]

Interesting, can we also write a simple test case for this?

[devvit]

@izelnakri, I reviewed current test codes, there are many hardcodes, it's difficult to create another string_repo for me, :(

But I can use an environment var STRING_TEST to complete test with uuid_repo, while STRING_TEST is unset, run uuid suite(and skip two tests), otherwise string.

The PR is imperfect, but simple