new scanf implementation and corresponding integer parser/converter

advantages over the old code:
- correct results for floating point (old code was bogus)
- wide/regular scanf separated so scanf does not pull in wide code
- well-defined behavior on integers that overflow dest type
- support for %[a-b] ranges with %[ (impl-defined by widely used)
- no intermediate conversion of fmt string to wide string
- cleaner, easier to share code with strto* functions
- better standards conformance for corner cases

the old code remains in the source tree, as the wide versions of the
scanf-family functions are still using it. it will be removed when no
longer needed.

src/internal/stdio_impl.h

@@ -69,6 +69,8 @@ size_t __stdout_write(FILE *, const unsigned char *, size_t);
 off_t __stdio_seek(FILE *, off_t, int);
 int __stdio_close(FILE *);
 
+size_t __string_read(FILE *, unsigned char *, size_t);
+
 int __toread(FILE *);
 int __towrite(FILE *);