Scaffold for sequence terminal

I realized that we can use our value block system to fix _most_ of the remaining 
order-of-evaluation issues we had with sequence expressions. This PR adds a new 
SequenceTerminal to HIR; there is already a ReactiveFunction equivalent 
(ReactiveSequenceValue) that the next PR will convert this terminal into.

compiler/forget/src/HIR/HIR.ts

@@ -279,7 +279,8 @@ export type Terminal =
   | LogicalTerminal
   | TernaryTerminal
   | OptionalTerminal
-  | LabelTerminal;
+  | LabelTerminal
+  | SequenceTerminal;
 
 function _staticInvariantTerminalHasLocation(
   terminal: Terminal
@@ -298,7 +299,6 @@ function _staticInvariantTerminalHasInstructionId(
 /**
  * Terminal nodes allowed for a value block
  */
-export type ValueTerminal = IfTerminal | GotoTerminal;
 // A terminal that couldn't be lowered correctly.
 export type UnsupportedTerminal = {
   kind: "unsupported";
@@ -437,6 +437,14 @@ export type OptionalTerminal = {
   loc: SourceLocation;
 };
 
+export type SequenceTerminal = {
+  kind: "sequence";
+  block: BlockId;
+  fallthrough: BlockId;
+  id: InstructionId;
+  loc: SourceLocation;
+};
+
 /**
  * Instructions generally represent expressions but with all nesting flattened away,
  * such that all operands to each instruction are either primitive values OR are

compiler/forget/src/HIR/HIRBuilder.ts

@@ -697,6 +697,10 @@ export function reversePostorderBlocks(func: HIR): void {
         visit(terminal.block);
         break;
       }
+      case "sequence": {
+        visit(terminal.block);
+        break;
+      }
       case "unsupported": {
         break;
       }

compiler/forget/src/HIR/PrintHIR.ts

@@ -79,6 +79,7 @@ export function printMixedHIR(
     return printInstruction(value);
   }
   switch (value.kind) {
+    case "sequence":
     case "label":
     case "optional":
     case "branch":
@@ -220,6 +221,10 @@ export function printTerminal(terminal: Terminal): Array<string> | string {
       }`;
       break;
     }
+    case "sequence": {
+      value = `[${terminal.id}] Sequence block=bb${terminal.block} fallthrough=bb${terminal.fallthrough}`;
+      break;
+    }
     case "unsupported": {
       value = `Unsupported`;
       break;

compiler/forget/src/HIR/visitors.ts

@@ -704,6 +704,17 @@ export function mapTerminalSuccessors(
         loc: terminal.loc,
       };
     }
+    case "sequence": {
+      const block = fn(terminal.block);
+      const fallthrough = fn(terminal.fallthrough);
+      return {
+        kind: "sequence",
+        block,
+        fallthrough,
+        id: makeInstructionId(0),
+        loc: terminal.loc,
+      };
+    }
     case "unsupported": {
       return terminal;
     }
@@ -737,6 +748,7 @@ export function terminalFallthrough(terminal: Terminal): BlockId | null {
     case "label":
     case "logical":
     case "optional":
+    case "sequence":
     case "switch":
     case "ternary":
     case "while": {
@@ -813,6 +825,10 @@ export function mapOptionalFallthroughs(
       }
       break;
     }
+    case "sequence": {
+      const _: BlockId = terminal.fallthrough;
+      break;
+    }
     default: {
       assertExhaustive(
         terminal,
@@ -881,6 +897,10 @@ export function* eachTerminalSuccessor(terminal: Terminal): Iterable<BlockId> {
       yield terminal.block;
       break;
     }
+    case "sequence": {
+      yield terminal.block;
+      break;
+    }
     case "unsupported":
       break;
     default: {
@@ -920,6 +940,7 @@ export function mapTerminalOperands(
       terminal.value = fn(terminal.value);
       break;
     }
+    case "sequence":
     case "label":
     case "optional":
     case "ternary":
@@ -967,6 +988,7 @@ export function* eachTerminalOperand(terminal: Terminal): Iterable<Place> {
       yield terminal.value;
       break;
     }
+    case "sequence":
     case "label":
     case "optional":
     case "ternary":

compiler/forget/src/ReactiveScopes/BuildReactiveFunction.ts

@@ -570,6 +570,7 @@ class Driver {
 
         break;
       }
+      case "sequence":
       case "optional":
       case "ternary":
       case "logical": {

compiler/forget/src/Utils/VisualizeHIRMermaid.ts

@@ -135,6 +135,10 @@ function printTerminalLabel(terminal: Terminal): string {
       buffer.push("Label");
       break;
     }
+    case "sequence": {
+      buffer.push("Sequence");
+      break;
+    }
     case "unsupported": {
       break;
     }
@@ -237,6 +241,11 @@ function printTerminalArrows(blockId: BlockId, terminal: Terminal): string {
       }
       break;
     }
+    case "sequence": {
+      buffer.push(printJumpArrow(blockId, terminal.block, "block"));
+      buffer.push(printJumpArrow(blockId, terminal.fallthrough, "fallthrough"));
+      break;
+    }
     case "throw":
     case "return":
     case "unsupported": {