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.

selfhost/cmd/w6l/out.ww

@@ -90,10 +90,10 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
 	wru64(hdr, 112u64, TEXT_OFF);		// p_align
 
 	// Write [0..0x1000) then .text.
-	let n1: i64 = os.writefull(fd, hdr, TEXT_OFF);
+	let n1: i64 = os.writeall(fd, hdr, TEXT_OFF);
 	if (n1 != TEXT_OFF: i64) { return -1; };
 	if (l.textlen > 0u64) {
-		let n2: i64 = os.writefull(fd, l.text, l.textlen);
+		let n2: i64 = os.writeall(fd, l.text, l.textlen);
 		if (n2 != l.textlen: i64) { return -1; };
 	};
 	return 0;