lib: rename stdlib surface to Hare names; add endian/math

Sweeping rename so the lib/ surface mirrors Hare's stdlib spellings.
- ascii: rune-taking predicates; ishex -> isxdigit
- bufio: rinit -> init; take1/takeline -> readbyte/readline
- bytes: indexsub -> index
- encoding/utf8: runelen -> runesz
- errors: eEOF/eShortRead/... -> eof/underread/...
- fmt: errln -> errorln; println/fprintln return i64
- os: readfull/writefull -> readall/writeall; unlink -> remove
- path: isabs -> abs; drop lastindex (now strings.rbyteindex)
- strconv: u64toa/i64toa -> u64tos/i64tos; parse64/parseu64 -> stoi64/stou64
- strings: drop len/isempty; equal -> compare; indexbyte -> byteindex; +rbyteindex
- types: drop numeric helpers (moved to math)
- new lib/endian (htonu16/ntohu16), lib/math (absi32/absi64)
- net: drop htons (use endian.htonu16)

Callers in selfhost/, lib/ww/, cmd/w6c/cgen.c, and test/wcc/700_e2e.c
updated to match.

lib/bytes/bytes.ww

@@ -30,9 +30,10 @@ export fn copy(dst: []u8, src: []u8) i32 = {
 	return n;
 };
 
-// indexsub — first index of `sub` in `s`, or -1. Mirrors
-// strings.index but on []u8. Empty `sub` matches at 0.
-export fn indexsub(s: []u8, sub: []u8) i32 = {
+// index — first index of `sub` in `s`, or -1. 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.
+export fn index(s: []u8, sub: []u8) i32 = {
 	if (sub.len == 0) { return 0; };
 	if (sub.len > s.len) { return -1; };
 	let last: i32 = s.len - sub.len;