vlc_tracer: pass types explicitly
The vlc_tracer supports 3 types:
- integer
- tick
- string
Key/value pairs are typically passed using the VLC_TRACE(key, value)
macro, where the type of the value implicitly determines the tracer
entry type.
One problem is that tick (vlc_tick_t) and integer (int64_t) have the
same C type, so there is no way to pass an integer value using this
macro.
Expose one VLC_TRACE_* function per type to avoid the problem.