Allow full path for tables in joinable! macro

[weiznich]

It's currently only possible to pass an identifier as table to the joinable! macro (and also the #[belongs_to] and #[has_many] annotations).
Possible:

use some_module::my_table_a;

joinable!(my_table_a -> my_table_b (some_field));

Not possible:

joinable!(some_module::my_table_a -> my_table_b (some_field));

Results in:

error: no rules expected the token `::`
  --> src/lib.rs:19:28
19 |> joinable!(some_module::my_table_a -> my_table_b (some_field));
   |>                      ^^

[Eijebong]

The problem is that we can't just change those ident by path as a path cannot be followed by a -> nor by a ( so it would require changing to something like joinable!(foo::bar => baz::lol { foo::bar::field })