GDNative: fix StringName equal and less operators

[Bromeon]

The new function accepts a pointer, where it constructs a StringName in-place:

void GDAPI godot_string_name_new(godot_string_name *r_dest, const godot_string *p_name) {
	StringName *dest = (StringName *)r_dest;
	const String *name = (const String *)p_name;
	memnew_placement(dest, StringName(*name));
}

This means that it's possible to have different pointers to the same StringName.

However, the equal and less methods only compare that pointer:

godot_bool GDAPI godot_string_name_operator_equal(const godot_string_name *p_self, const godot_string_name *p_other) {
	const StringName *self = (const StringName *)p_self;
	const StringName *other = (const StringName *)p_other;
	return self == other;
}

godot_bool GDAPI godot_string_name_operator_less(const godot_string_name *p_self, const godot_string_name *p_other) {
	const StringName *self = (const StringName *)p_self;
	const StringName *other = (const StringName *)p_other;
	return self < other;
}

And to do that, they wouldn't need to first cast to StringName*. It looks like forgotten dereferencing.

[Bromeon]

I haven't had the chance to test this change in godot-rust yet, hopefully will get around to it later today.

[akien-mga]

Makes sense to me. I'll wait for feedback on how it fares in godot-rust to merge.

[Bromeon]

Added tests in godot-rust/gdnative#912, they are currently failing as expected.

When I compile Godot locally with the change in this PR, they pass. I couldn't use this exact revision since godot-rust is still based on Godot 3.4, so I cherry-picked my commit from here onto an older Godot revision (for my local tests).

TLDR: ready from my side.

[akien-mga]

Thanks!