Prefix filenames with rbs_

[st0012]

Most of the header/source files are now prefixed with rbs_, so @amomchilov and I think it'd better to have them all prefixed that way too.

[st0012]

In fact, cpp shouldn't have any conflicts based on just file names, which I've verified through a small experiment:

I have both rbs/util/rbs_assert.h and rbs/rbs_assert.h

#ifndef RBS_ASSERT_H
#define RBS_ASSERT_H

#include "rbs/rbs_defines.h"
#include <stdbool.h>

void rbs_assert(bool condition, const char *fmt, ...) RBS_ATTRIBUTE_FORMAT(2, 3);

#endif

#ifndef RBS_ROOT_ASSERT_H
#define RBS_ROOT_ASSERT_H

#include "rbs/rbs_defines.h"
#include <stdbool.h>

void rbs_assert_2(bool condition, const char *fmt, ...) RBS_ATTRIBUTE_FORMAT(2, 3);

#endif

And include both of them:

#include "rbs/rbs_encoding.h"
#include "rbs/rbs_assert.h"
#include "rbs/util/rbs_assert.h"

And it compiles.