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.

cmd/w6c/cgen.c

@@ -790,7 +790,7 @@ cgexpr(Cg *c, Node *n, Local *locals)
 			/* Use DIV (unsigned) when either operand is an unsigned
 			 * integer type — IDIV would sign-extend a u64 with high
 			 * bit set into a negative i64 and produce wrong results
-			 * (see strconv.u64toa with v = 1 << 63). */
+			 * (see strconv.u64tos with v = 1 << 63). */
 			int unsignd = (n->lhs && type_isunsigned(n->lhs->type))
 			    || (n->rhs && type_isunsigned(n->rhs->type));
 			ins2(c, A_MOVQ, aimm(0), areg(D_DX));
@@ -819,7 +819,7 @@ cgexpr(Cg *c, Node *n, Local *locals)
 			/* For ordered comparisons on unsigned operands we must
 			 * use the JA/JAE/JB/JBE family — signed Jcc would treat
 			 * a u64 with the high bit set as negative (e.g. the
-			 * loop guard `n > 0` in strconv.u64toa with n=1<<63). */
+			 * loop guard `n > 0` in strconv.u64tos with n=1<<63). */
 			int unsignd = (n->lhs && type_isunsigned(n->lhs->type))
 			    || (n->rhs && type_isunsigned(n->rhs->type));
 			ins2(c, A_CMPQ, areg(D_BX), areg(D_AX));