Hygenie issue in `ascent_src!`

[StarGazerM]

ascent_src! don't have capture ability for var defined outside src due to macro hygenie issue. Maybe we can consider allow declaration of captured var in syntax?

Code failed now:

#[test]
fn test_macro() {
   let z = 1;
   mod foom {
      use super::*;
      ascent_source! {
         foo_gen ():
         foo(x, y) <-- foo(y, x), let _ = z;
      }
   }
   ascent_run! {
      // struct MacroTest;
      relation foo(usize, usize);
      include_source!(foom::foo_gen);
   };
}

error message:

error[E0425]: cannot find value `z` in this scope
  --> ascent/examples/ascent_source.rs:27:43
   |
27 |          foo(x, y) <-- foo(y, x), let _ = z;
   |                                           ^ help: a local variable with a similar name exists: `x`
...
33 |       include_source!(foom::foo_gen);
   |                       ------------- in this macro invocation
   |
   = note: this error originates in the macro `foom::foo_gen` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0425`.
error: could not compile `ascent` (example "ascent_source" test) due to 1 previous error