object: speed up type verification by not relying on strcmp()

Instead of using string contents for type identification use the address
of a constant string array. This should speed up type verifications a
little sind we only need to compare one machine word instead of a full
string. Also, this saves a few strings.

To make clear that types must be compared via address and not string
contents 'type_name' is now called 'type_id'.

This also simplifies the macros for declaring and defining public and
private subclasses.

src/pulsecore/source.h

@@ -158,7 +158,7 @@ struct pa_source {
     void *userdata;
 };
 
-PA_DECLARE_CLASS(pa_source);
+PA_DECLARE_PUBLIC_CLASS(pa_source);
 #define PA_SOURCE(s) pa_source_cast(s)
 
 typedef enum pa_source_message {