Implement support for sys.path when loading modules.

sys.path is not initialized by rt_init(), that's left for platform-specific
startup code. (For example, bare metal port may have some hardcoded defaults,
and let user change sys.path directly; while port for OS with environment
feature can take path from environment). If it's not explicitly initialized,
modules will be imported only from a current directory.

py/misc.h

@@ -90,6 +90,17 @@ void vstr_add_strn(vstr_t *vstr, const char *str, int len);
 void vstr_cut_tail(vstr_t *vstr, int len);
 void vstr_printf(vstr_t *vstr, const char *fmt, ...);
 
+/** non-dynamic size-bounded variable buffer/string *************/
+
+#define CHECKBUF(buf, max_size) char buf[max_size + 1]; uint buf##_len = max_size; char *buf##_p = buf;
+#define CHECKBUF_APPEND(buf, src, src_len) \
+        { int l = MIN(src_len, buf##_len); \
+        memcpy(buf##_p, src, l); \
+        buf##_len -= l; \
+        buf##_p += l; }
+#define CHECKBUF_APPEND_0(buf) { *buf##_p = 0; }
+#define CHECKBUF_LEN(buf) (buf##_p - buf)
+
 #ifdef va_start
 void vstr_vprintf(vstr_t *vstr, const char *fmt, va_list ap);
 #endif