Eliminate GuardedTypeConversion, DynamicLinker.getCurrentLinkRequest and its associated permission

Eliminate both GuardedTypeConversion and DynamicLinker.getCurrentLinkRequest and its associated permission. Instead introduce a more robust scheme where GuardingTypeConverterFactory.convertToType takes a Supplier<MethodHandles.Lookup> parameter through which it can access the current lookup when needed, subject to the CallSiteDescriptor.getLookup security check. This also allows a more robust design where the TypeConverterFactory can decide on whether a converter is lookup-sensitive and thus not cacheable based on whether any of the converter factories actually retrieved the lookup object through the supplier; previously the factories had to be trusted with this decision by setting the GuardedTypeConversion.cacheable flag.