Experimental control-flow operator (#368)

* AST and syntax for delimited control

* stashing for later

* a little more progress

* progress on delimited continuations

* delimit, suspend, and resume implemented (draft)

* example that generates the natural numbers

* fixes

* tinkering

* changed demo to experimental

* comments and name changes

* describe delimited continuations in the README

* renamed Snapshot to Continuation, edits to comments

* Update executable_semantics/ast/statement.h

improve comment for MakeDelimitStmt

Co-authored-by: Dave Abrahams <dabrahams@google.com>

* Update executable_semantics/interpreter/interpreter.cpp

remove snake_case

Co-authored-by: Dave Abrahams <dabrahams@google.com>

* edits to comments, change name of variable

* updates to handle review edits

* trailing whitespace

* fixes to delimited continuations, added more tests, also fixed assignment to do a copy

* improvements from Geoffrey

* new test from Geoffrey, fix for empty blocks

* more suggestions from Geoffrey

* more tests for delimited continuations, renaming some of them

* renamed test files

* improve a comment

* sketch of creating continuation

* initial implementation of shift/reset style continuations

* more documentation

* fix some camel case

* implemented deep copy of continuations, added a test case for it

* fixed a bug and got the recursive test case working

* removed __delimit, polished up __continuation

* back to shallow copy for continuations

* suggestions from Geoffrey

* removed structured binding (for now)

* Update executable_semantics/ast/expression.cpp

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

* responses to Geoffrey

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

executable_semantics/ast/expression.cpp

@@ -36,6 +36,14 @@ auto MakeAutoType(int line_num) -> Expression* {
   return t;
 }
 
+// Returns a Continuation type AST node at the given source location.
+auto MakeContinuationType(int line_num) -> Expression* {
+  auto* type = new Expression();
+  type->tag = ExpressionKind::ContinuationT;
+  type->line_num = line_num;
+  return type;
+}
+
 auto MakeFunType(int line_num, Expression* param, Expression* ret)
     -> Expression* {
   auto* t = new Expression();
@@ -150,6 +158,18 @@ auto MakeTuple(int line_num,
   return e;
 }
 
+// Create an AST node for an empty tuple.
+// TODO(geoffromer): remove this and rewrite its callers to use
+// `MakeTuple(line_num, {})`, once that works.
+auto MakeUnit(int line_num) -> Expression* {
+  auto* unit = new Expression();
+  unit->line_num = line_num;
+  unit->tag = ExpressionKind::Tuple;
+  auto* args = new std::vector<std::pair<std::string, Expression*>>();
+  unit->u.tuple.fields = args;
+  return unit;
+}
+
 auto MakeIndex(int line_num, Expression* exp, Expression* i) -> Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
@@ -171,13 +191,13 @@ static void PrintOp(Operator op) {
       std::cout << "-";
       break;
     case Operator::Not:
-      std::cout << "!";
+      std::cout << "not";
       break;
     case Operator::And:
-      std::cout << "&&";
+      std::cout << "and";
       break;
     case Operator::Or:
-      std::cout << "||";
+      std::cout << "or";
       break;
     case Operator::Eq:
       std::cout << "==";
@@ -272,6 +292,9 @@ void PrintExp(const Expression* e) {
     case ExpressionKind::AutoT:
       std::cout << "auto";
       break;
+    case ExpressionKind::ContinuationT:
+      std::cout << "Continuation";
+      break;
     case ExpressionKind::FunctionT:
       std::cout << "fn ";
       PrintExp(e->u.function_type.parameter);