lib: drop non-Hare extras (ascii.digitval, bytes.copy, fmt.errpos)

ascii.digitval/isidstart/isidpart are lexer-private, not Hare-stdlib.
Moved into lib/ww/lex/lex.ww as fn (renamed digitval to hexval since
its sole job is the \\xHH escape decoder).

bytes.copy and fmt.errpos have no Hare counterpart and no external
callers; gone.

lib/bytes/bytes.ww

@@ -21,17 +21,6 @@ export fn indexbyte(s: []u8, c: u8) (i32 | void) = {
 	return;
 };
 
-export fn copy(dst: []u8, src: []u8) i32 = {
-	let n: i32 = dst.len;
-	if (src.len < n) { n = src.len; };
-	let i: i32 = 0;
-	for (i < n) {
-		dst[i] = src[i];
-		i += 1;
-	};
-	return n;
-};
-
 // index — first index of `sub` in `s`. Mirrors Hare's bytes::index
 // (the []u8 needle variant; the u8 needle stays as indexbyte until we
 // have union-arg dispatch). Empty `sub` matches at 0.