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/net/net.ww

@@ -42,7 +42,6 @@ export fn listen(fd: i32, backlog: i32) i32 = {
 	return syscall3(SYS_LISTEN, fd: i64, backlog: i64, 0): i32;
 };
 
-// htons-equivalent: byte-swap a 16-bit port into network order.
-export fn htons(p: u16) u16 = {
-	return ((p << 8) | (p >> 8)) & 0xffff;
-};
+// Byte-order conversions live in lib/endian (Hare's endian::htonu16
+// is the canonical name). Use `endian.htonu16` to put a port into
+// network order.