Make source compilable with C++17

Changes applied:

- Removing "register" keyword
- Fixing const pointer to pointer assignments
- Adding type casts to rb_malloc calls

src/string.c

@@ -2835,8 +2835,8 @@ mrb_float_read(const char *string, char **endPtr)
     int sign, expSign = FALSE;
     double fraction, dblExp;
     const double *d;
-    register const char *p;
-    register int c;
+    const char *p;
+    int c;
     int exp = 0;                /* Exponent read from "EX" field. */
     int fracExp = 0;            /* Exponent that derives from the fractional
                                  * part.  Under normal circumstatnces, it is