Fix arena_sb_append_cstr by adding arena_strlen forward declaration

[savashn]

Hello. This PR fixes implicit declaration error below by adding a forward declaration for arena_strlen(). Otherwise, calling #define ARENA_IMPLEMENTATIN in every .c file causes multiple definition errors.

In file included from main.c:2:
main.c: In function ‘main’:
arena.h:136:20: error: implicit declaration of function ‘arena_strlen’; did you mean ‘arena_strdup’? [-Wimplicit-function-declaration]
  136 |         size_t n = arena_strlen(s);        \
      |                    ^~~~~~~~~~~~
main.c:15:5: note: in expansion of macro ‘arena_sb_append_cstr’
   15 |     arena_sb_append_cstr(&arena, &sb, "Hello, ");
      |     ^~~~~~~~~~~~~~~~~~~~

[savashn]

Hi @rexim, sorry for pinning. I’ve updated the PR description to clarify the change. Just wanted to check if you had a chance to review this. Have a nice day.