Organize the include of header files

- `#include <math.h>` is done in `mruby.h`.
  Eliminate the need to worry about the `MRB_NO_FLOAT` macro.

- Include mruby header files before standard header files.
  If the standard header file is already placed before `mruby.h`, the standard header file added in the future tends to be placed before `mruby.h`.

This change should some reduce the chances of macros that must be defined becoming undefined in C++ or including problematic header files in a particular mruby build configuration.

src/string.c

@@ -15,9 +15,6 @@
 #include <mruby/string.h>
 #include <mruby/numeric.h>
 #include <mruby/presym.h>
-#ifndef MRB_NO_FLOAT
-#include <math.h>
-#endif
 #include <string.h>
 
 typedef struct mrb_shared_string {