Individualize NameHashSet Hashing & Revisit #709 (#740)

* Make mac/win lookups lowercase'd

* Revert #709 & 81042cc

* Adjust HashPolicy implementation across sourcemod

Basically, in order to implement our own (actual) hash policy in
`PluginSys.h`, we needed to remove the blanket implementation of `hash`
that was used before. Now, each policy must implement `hash` along with
`matches` in order to be used with `NameHashSet`. While this does force
us to change every implementation of policies across the entirety of
sourcemod, it allows core to use flexible implementations of `hash`.

* Remove logic duplication

* Improve lowercase checks

core/ConVarManager.h

@@ -43,6 +43,7 @@
 #include <compat_wrappers.h>
 #include "concmd_cleaner.h"
 #include "PlayerManager.h"
+#include <sm_stringhashmap.h>
 
 using namespace SourceHook;
 
@@ -67,6 +68,10 @@ struct ConVarInfo
 	{
 		return strcmp(name, info->pVar->GetName()) == 0;
 	}
+	static inline uint32_t hash(const detail::CharsAndLength &key)
+	{
+		return key.hash();
+	}
 };
 
 /**