Convert Statement to use Ptr (#788)

Note this makes a few cases where the Statement was optional explicit (Block, If, Sequence). I do add a few CHECKs around where statements were optional and assumed but unverified.

I switch TypeCheckStmt to not take an optional Statement because I think it makes the call sites clearer in behavior. It's also a smaller change than the converse, because taking an optional Statement means the returned statement would also need to be optional. Arguably a wrapper for optional statements could be added, but this still seems cleaner to me, and there aren't that many cases of an optional statement.

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

executable_semantics/ast/function_definition.cpp

@@ -27,7 +27,7 @@ void FunctionDefinition::PrintDepth(int depth, llvm::raw_ostream& out) const {
   }
   if (body) {
     out << " {\n";
-    body->PrintDepth(depth, out);
+    (*body)->PrintDepth(depth, out);
     out << "\n}\n";
   } else {
     out << ";\n";