Explorer and toolchain changes to implement #2483 (#2707)

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.

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Co-authored-by: Geoff Romer <gromer@google.com>

explorer/interpreter/matching_impl_set.cpp

@@ -105,7 +105,7 @@ auto MatchingImplSet::GetLabelForDeclaration(const Declaration& declaration)
 }
 
 MatchingImplSet::Match::Match(Nonnull<MatchingImplSet*> parent,
-                              Nonnull<const ImplScope::Impl*> impl,
+                              Nonnull<const ImplScope::ImplFact*> impl,
                               Nonnull<const Value*> type,
                               Nonnull<const Value*> interface)
     : parent_(parent), impl_(impl), type_(type), interface_(interface) {