Use "construct" instead of "this" to define constructors.

test/limit/many_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new() {
+  construct new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3

test/limit/many_inherited_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new() {
+  construct new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3
@@ -137,7 +137,7 @@ class Foo {
 }
 
 class Bar is Foo {
-  this new() {
+  construct new() {
     super()
     _field129 = 129
     _field130 = 130

test/limit/too_many_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new() {
+  construct new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3

test/limit/too_many_inherited_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new() {
+  construct new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3
@@ -137,7 +137,7 @@ class Foo {
 }
 
 class Bar is Foo { // expect runtime error: Class 'Bar' may not have more than 255 fields, including inherited ones.
-  this new() {
+  construct new() {
     super()
     _field129 = 129
     _field130 = 130