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/syntax/parser.ypp

@@ -110,6 +110,10 @@ void yy::parser::error(
 %token IF
 %token ELSE
 %token WHILE
+%token CONTINUATION_TYPE
+%token CONTINUATION
+%token RUN
+%token AWAIT
 %token BREAK
 %token CONTINUE
 %token RETURN
@@ -180,6 +184,8 @@ expression:
     { $$ = Carbon::MakeTypeType(yylineno); }
 | AUTO
     { $$ = Carbon::MakeAutoType(yylineno); }
+| CONTINUATION_TYPE
+    { $$ = Carbon::MakeContinuationType(yylineno); }
 | paren_expression { $$ = $1; }
 | expression EQUAL_EQUAL expression
     { $$ = Carbon::MakeBinOp(yylineno, Carbon::Operator::Eq, $1, $3); }
@@ -287,6 +293,12 @@ statement:
     { $$ = Carbon::MakeBlock(yylineno, $2); }
 | MATCH "(" expression ")" "{" clause_list "}"
     { $$ = Carbon::MakeMatch(yylineno, $3, $6); }
+| CONTINUATION identifier statement
+    { $$ = Carbon::MakeContinuationStatement(yylineno, $2, $3); }
+| RUN expression ";"
+    { $$ = Carbon::MakeRun(yylineno, $2); }
+| AWAIT ";"
+    { $$ = Carbon::MakeAwait(yylineno); }
 ;
 optional_else:
   // Empty