LibJS: Fast non-local variable access :^)

This patch introduces the "environment coordinate" concept, which
encodes the distance from a variable access to the binding it ends up
resolving to.

EnvironmentCoordinate has two fields:

    - hops:  The number of hops up the lexical environment chain we have
             to make before getting to the resolved binding.

    - index: The index of the resolved binding within its declarative
             environment record.

Whenever a variable lookup resolves somewhere inside a declarative
environment, we now cache the coordinates and reuse them in subsequent
lookups. This is achieved via a coordinate cache in JS::Identifier.

Note that non-strict direct eval() breaks this optimization and so it
will not be performed if the resolved environment has been permanently
screwed by eval().

This makes variable access *significantly* faster. :^)

Userland/Libraries/LibJS/AST.h

@@ -17,6 +17,7 @@
 #include <AK/Variant.h>
 #include <AK/Vector.h>
 #include <LibJS/Forward.h>
+#include <LibJS/Runtime/EnvironmentCoordinate.h>
 #include <LibJS/Runtime/PropertyName.h>
 #include <LibJS/Runtime/Reference.h>
 #include <LibJS/Runtime/Value.h>
@@ -984,6 +985,7 @@ private:
     virtual bool is_identifier() const override { return true; }
 
     FlyString m_string;
+    mutable Optional<EnvironmentCoordinate> m_cached_environment_coordinate;
 };
 
 class ClassMethod final : public ASTNode {