Fix IntoIterator derived bounds (#208)

[MegaBluejay]

Resolves #208

Synopsis

For generic structs, some derived IntoIterator bounds are based on the struct's type parameters, rather than the field used in the implementation.

Solution

Require only the type of the field used to be IntoIterator

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

Note

I don't think this is a breaking change, since no program where the newly added bounds aren't satisfied would have compiled.

[tyranron]

Tests are still failing.

[tyranron]

@MegaBluejay such unification is not good for tests, as doesn't allow us to understand well which assertion in the test case is failing exactly:

---- generic stdout ----
thread 'generic' panicked at 'assertion failed: `(left == right)`
  left: `[1, 2, 3]`,
 right: `[]`', tests/into_iterator.rs:18:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- generic_bounds stdout ----
thread 'generic_bounds' panicked at 'assertion failed: `(left == right)`
  left: `[1, 2, 3]`,
 right: `[]`', tests/into_iterator.rs:18:5

---- generic_refs stdout ----
thread 'generic_refs' panicked at 'assertion failed: `(left == right)`
  left: `[1, 2, 3]`,
 right: `[]`', tests/into_iterator.rs:18:5

---- generic_owned stdout ----
thread 'generic_owned' panicked at 'assertion failed: `(left == right)`
  left: `[1, 2, 3]`,
 right: `[]`', tests/into_iterator.rs:18:5

---- named_single stdout ----
thread 'named_single' panicked at 'assertion failed: `(left == right)`
  left: `[1, 2, 3]`,
 right: `[]`', tests/into_iterator.rs:18:5

---- tuple_single stdout ----
thread 'tuple_single' panicked at 'assertion failed: `(left == right)`
  left: `[1, 2, 3]`,
 right: `[]`', tests/into_iterator.rs:18:5

#[track_caller] may be helpful here to some degree, but not when multiple assertions are united in the same helper function.