Fix type param bounds

[brasswood]

fixes #384

[kevinmehall]

Can you add a test in tests/run-pass for these return types?

[brasswood]

How do I check if my test compiles?

[kevinmehall]

cargo test will run all tests, or cargo test -- trybuild trybuild=somefile.rs to run only a specific file.

[brasswood]

This is WIP, I need to somehow compare a trait object to something else

[kevinmehall]

Since your traits all require Debug, you could format!("{:?}", iter.next()) the trait objects and compare the resulting string.

[brasswood]

Should users be allowed to write something like grammar assembly<'b, 'input: 'b>() for str? i.e. constrain some lifetime parameter to make sure 'input outlives it? I tried it and it gave me the error:

error: expected one of ",", ">"
   --> tests/run-pass/assembly_ast_dyn_type_param_bounds.rs:110:28
    |
110 | grammar assembly<'b, 'input: 'b>() for str {
    |                            ^