libbacktrace Vala binding

libbacktrace.vapi

```vala
[CCode (cprefix = "BACKTRACE_", lower_case_cprefix = "BACKTRACE_", cheader_filename = "backtrace.h,backtrace-supported.h")]
namespace Backtrace {
    public const int SUPPORTED;

    public const int USES_MALLOC;

    public const int SUPPORTS_THREADS;

    public const int SUPPORTS_DATA;

    [CCode (cname = "backtrace_error_callback", has_target = false)]
    public delegate void ErrorCallback (void* data, string msg, int errnum);

    [CCode (cname = "backtrace_full_callback", has_target = false)]
    public delegate int FullCallback (void* data, uintptr pc, string? filename, int lineno, string function);

    [CCode (cname = "backtrace_simple_callback", has_target = false)]
    public delegate int SimpleCallback (void* data, uintptr pc);

    [CCode (cname = "backtrace_syminfo_callback", has_target = false)]
    public delegate void SyminfoCallback (void* data, uintptr pc, string symname, uintptr symval, uintptr symsize);

    [Compact]
	[CCode (cname = "struct backtrace_state", cprefix = "backtrace_", has_destroy_function = false)]
	public class State {
        [CCode (cname = "backtrace_create_state")]
        public State (string filename, int threaded, ErrorCallback error_callback, void* data);
        
        [CCode (instance_pos = 0)]
        public int full(int skip, FullCallback callback,  ErrorCallback error_callback, void* data);

        [CCode (instance_pos = 0)]
        public int simple (int skip, SimpleCallback callback, ErrorCallback error_callback, void* data);

        [CCode (instance_pos = 0)]
        public void print (int skip, GLib.FileStream file = GLib.stderr);

        [CCode (instance_pos = 0)]
        public int pcinfo (uintptr pc, FullCallback callback, ErrorCallback error_callback, void* data);

        [CCode (instance_pos = 0)]
        public int syminfo (uintptr addr, SyminfoCallback callback, ErrorCallback error_callback, void* data);
    }
}
```