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/ast/declaration.cpp

@@ -45,7 +45,8 @@ void Declaration::Print(llvm::raw_ostream& out) const {
       const auto& match_first_decl = cast<MatchFirstDeclaration>(*this);
       PrintID(out);
       out << " {\n";
-      for (Nonnull<const ImplDeclaration*> m : match_first_decl.impls()) {
+      for (Nonnull<const ImplDeclaration*> m :
+           match_first_decl.impl_declarations()) {
         out << *m;
       }
       out << "}\n";