This allows to use MappedBuffer and similar types to be properly
debug-printed.
Also change VideoFrame/VideoFrameRef/RTPBuffer from a tuple struct to a
struct with proper field names to make the code easier to understand.
They can actually be shared with multiple threads at the same time
safely as all functions requiring an immutable reference are
thread-safe.
OTOH TypeFind can't be shared safely between different threads as not
all implementations of the TypeFind struct are thread-safe.
