Make source mapping use byte offset instead of line and col

for better debugging support in repl. Add debug module for better
debugging support.

src/core/corelib.c

@@ -801,6 +801,7 @@ JanetTable *janet_core_env(void) {
         janet_lib_os(args);
         janet_lib_parse(args);
         janet_lib_compile(args);
+        janet_lib_debug(args);
         janet_lib_string(args);
         janet_lib_marsh(args);
 #ifdef JANET_ASSEMBLER