Make FunctionDescriptor::toMethodType an instance method of Linker

Having a single way to map a function descriptor into a method type seems overly restrictive. After all, any given linker implementation can have its own opinion on how layouts should be mapped into carriers. For instance, while a native linker might want to map `AddressLayout` into `MemorySegment`, a JNI linker might want to map the same layout into `Object` (as JNI only supports passing Java objects as references).