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/w6c/main.ww

@@ -46,7 +46,7 @@ fn slurp(path: *u8) (*u8, u64) = {
 	if (n < 0i64) { os.close(fd); return nil, 0u64; };
 	let nz: u64 = n: u64;
 	let buf: *u8 = os.alloc(nz + 1u64): *u8;
-	let got: i64 = os.readfull(fd, buf, nz);
+	let got: i64 = os.readall(fd, buf, nz);
 	os.close(fd);
 	if (got != n) { return nil, 0u64; };
 	buf[nz] = 0u8;