[hir][be] Use a Map to store ObjectExpression.properties

Semantically this seems like a better fit as we're using Map like methods to 
iterate and update values anyway.

compiler/forget/src/HIR/BuildHIR.ts

@@ -682,7 +682,7 @@ function lowerExpression(
     case "ObjectExpression": {
       const expr = exprPath as NodePath<t.ObjectExpression>;
       const propertyPaths = expr.get("properties");
-      const properties: { [name: string]: Place } = {};
+      const properties: Map<string, Place> = new Map();
       for (const propertyPath of propertyPaths) {
         todoInvariant(
           propertyPath.isObjectProperty(),
@@ -696,7 +696,7 @@ function lowerExpression(
           "Handle non-expression object values"
         );
         const value = lowerExpressionToPlace(builder, valuePath);
-        properties[key.name] = value;
+        properties.set(key.name, value);
       }
       return {
         kind: "ObjectExpression",

compiler/forget/src/HIR/Codegen.ts

@@ -200,7 +200,7 @@ function writeInstr(cx: Context, instr: Instruction, body: Array<t.Statement>) {
     case "ObjectExpression": {
       const properties = [];
       if (instrValue.properties !== null) {
-        for (const [property, value] of Object.entries(instrValue.properties)) {
+        for (const [property, value] of instrValue.properties) {
           properties.push(
             t.objectProperty(t.stringLiteral(property), codegenPlace(cx, value))
           );

compiler/forget/src/HIR/HIR.ts

@@ -190,7 +190,7 @@ export type InstructionData =
     }
   | {
       kind: "ObjectExpression";
-      properties: { [property: string]: Place } | null; // null === empty object
+      properties: Map<string, Place> | null; // null === empty object
     }
   | { kind: "ArrayExpression"; elements: Array<Place> }
 

compiler/forget/src/HIR/InferReferenceEffects.ts

@@ -526,7 +526,7 @@ function inferBlock(env: Environment, block: BasicBlock) {
         valueKind = ValueKind.Mutable;
         // Object construction captures but does not modify the key/property values
         if (instrValue.properties !== null) {
-          for (const [_key, value] of Object.entries(instrValue.properties)) {
+          for (const [_key, value] of instrValue.properties) {
             env.reference(value, Effect.Read);
           }
         }

compiler/forget/src/HIR/PrintHIR.ts

@@ -161,7 +161,7 @@ function printInstructionValue(instrValue: InstructionValue): string {
     case "ObjectExpression": {
       const properties = [];
       if (instrValue.properties !== null) {
-        for (const [key, value] of Object.entries(instrValue.properties)) {
+        for (const [key, value] of instrValue.properties) {
           properties.push(`${key}: ${printPlace(value)}`);
         }
       }