implement wcstod and family

not heavily tested but these functions appear to work correctly

src/stdlib/wcstod.c

+#include "shgetc.h"
+#include "floatscan.h"
+#include "stdio_impl.h"
+
+/* This read function heavily cheats. It knows:
+ *  (1) len will always be 1
+ *  (2) non-ascii characters don't matter */
+
+static size_t do_read(FILE *f, unsigned char *buf, size_t len)
+{
+	size_t i;
+	const wchar_t *wcs = f->cookie;
+
+	if (!wcs[0]) wcs=L"@";
+	for (i=0; i<f->buf_size && wcs[i]; i++)
+		f->buf[i] = wcs[i] < 128 ? wcs[i] : '@';
+	f->rpos = f->buf;
+	f->rend = f->buf + i;
+	f->cookie = (void *)(wcs+i);
+
+	if (i && len) {
+		*buf = *f->rpos++;
+		return 1;
+	}
+	return 0;
+}
+
+static long double wcstox(const wchar_t *s, wchar_t **p, int prec)
+{
+	wchar_t *t = (wchar_t *)s;
+	unsigned char buf[64];
+	FILE f = {0};
+	f.flags = 0;
+	f.rpos = f.rend = 0;
+	f.buf = buf + 4;
+	f.buf_size = sizeof buf - 4;
+	f.lock = -1;
+	f.read = do_read;
+	while (iswspace(*t)) t++;
+	f.cookie = (void *)t;
+	shlim(&f, 0);
+	long double y = __floatscan(&f, -1, prec, 1);
+	off_t cnt = shcnt(&f);
+	if (p) *p = cnt ? t + cnt : (wchar_t *)s;
+	return y;
+}
+
+float wcstof(const wchar_t *s, wchar_t **p)
+{
+	return wcstox(s, p, 0);
+}
+
+double wcstod(const wchar_t *s, wchar_t **p)
+{
+	return wcstox(s, p, 1);
+}
+
+long double wcstold(const wchar_t *s, wchar_t **p)
+{
+	return wcstox(s, p, 2);
+}