adds trailing comma to single element tuple
example:
```carbon
package ExplorerTest api;
fn Main() -> i32
{
var a: (i32,) = (1, 2);
return 0;
}
```
change in error message:
```diff
-type error in initializer of variable: '(i32, i32)' is not implicitly convertible to '(i32)'
+type error in initializer of variable: '(i32, i32)' is not implicitly convertible to '(i32,)'
```
This PR is making two main changes to the Explorer and Toolchain:
- Replace the `is` keyword in `where SomeType is SomeInterface` with `impls`, so it is `where SomeType impls SomeInterface`
- Rewrite uses of the "impls" to something else to avoid, frequently "`impl` declarations" or "implementations", to avoid confusion with the `impls` keyword.
---------
Co-authored-by: Geoff Romer <gromer@google.com>
Both cases need a label for each level of struct and tuple type appearing in the impl query, because we can have an unbounded number of such levels with the types otherwise being the same.
Prior to this, both added testcases exhibit unbounded recursion.
Manmeet Singh
josh11bandGeoff Romer
Richard Smith