analogous to commit b287cd745c2243f8e5114331763a5a9813b5f6ee but for
the custom FILE stream type the wcstol and wcstod family use. __toread
could be used here as well, but there's a simple direct fix to make
the buffer pointers initially valid for subtraction, so just do that
to avoid pulling in stdio exit code in programs that don't use stdio.

this header evolved to facilitate the extremely lazy practice of
omitting explicit includes of the necessary headers in individual
stdio source files; not only was this sloppy, but it also increased
build time.
now, stdio_impl.h is only including the headers it needs for its own
use; any further headers needed by source files are included directly
where needed.

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

not heavily tested but these functions appear to work correctly