LibJS: Shrink Identifier's environment coordinate cache

This patch does two things:

- We now use u32 instead of size_t for the hops and index fields
  in EnvironmentCoordinate. This means we're limited to an environment
  nesting level and variable count of 4Gs respectively.

- Instead of wrapping it in an Optional, EnvironmentCoordinate now has
  a custom valid/invalid state using a magic marker value.

These two changes reduce the size of Identifier by 16 bytes. :^)

Userland/Libraries/LibJS/Runtime/EnvironmentCoordinate.h

@@ -12,10 +12,13 @@
 namespace JS {
 
 struct EnvironmentCoordinate {
-    size_t hops { 0 };
-    size_t index { 0 };
+    u32 hops { invalid_marker };
+    u32 index { invalid_marker };
 
-    static constexpr size_t global_marker = 0xffffffff;
+    bool is_valid() const { return hops != invalid_marker && index != invalid_marker; }
+
+    static constexpr u32 global_marker = 0xffffffff;
+    static constexpr u32 invalid_marker = 0xfffffffe;
 };
 
 }

Userland/Libraries/LibJS/Runtime/VM.cpp

@@ -565,8 +565,11 @@ ThrowCompletionOr<Reference> VM::get_identifier_reference(Environment* environme
 
     // Note: This is an optimization for looking up the same reference.
     Optional<EnvironmentCoordinate> environment_coordinate;
-    if (index.has_value())
-        environment_coordinate = EnvironmentCoordinate { .hops = hops, .index = index.value() };
+    if (index.has_value()) {
+        VERIFY(hops <= NumericLimits<u32>::max());
+        VERIFY(index.value() <= NumericLimits<u32>::max());
+        environment_coordinate = EnvironmentCoordinate { .hops = static_cast<u32>(hops), .index = static_cast<u32>(index.value()) };
+    }
 
     // 3. If exists is true, then
     if (exists) {