lib/strconv: graduate to owned-str returns with Hare-shape base param

i64tos / u64tos / f64tos return a fresh owned str (caller frees via
os.free) instead of writing into a caller-supplied [N]u8. Adds typed
variants (i32tos / i16tos / i8tos and u32 / u16 / u8) and the missing
base parameter on stoi64 / stou64 + typed parse wrappers.

Base values are exported as plain-i32 `def`s (strconv.DEC,
strconv.HEX_UPPER, ...) rather than a `base` enum: cross-module
`strconv.base.DEC` chains miscompile in the cstage cgen — it emits a
memory load through `base(SB)` rather than inlining the constant.
The Sdef path resolves correctly, so callers say `strconv.DEC` and
both cgens lower to an immediate.

Also renames strings.byteindex / rbyteindex to strings.indexbyte /
rindexbyte, matching bytes.indexbyte and reserving the Hare name
`byteindex` for the future `(str | rune)`-needle shape.

fmt drops printint / printlnint / fprintint — those were stand-ins
for variadic `fmt::println(42)`; with the owned-str graduation the
substitute is one call: `fmt.println(strconv.i64tos(42, strconv.DEC))`.

strerror is sketched in a comment but not shipped — match arms over
the wider `error = !(invalid | overflow)` union still expose a
cstage-vs-wwstage spill divergence.

selfhost/test/smoke.ww

@@ -130,11 +130,10 @@ export fn main() i32 = {
 	if (dn != 3) { return 10; };
 
 	// Probe 5 — strconv round-trip via the real stdlib.
-	let outbuf: [32]u8;
-	let nb: i32 = strconv.i64tos(outbuf[0:32], 4242i64);
-	if (nb != 4) { return 11; };
-	if (outbuf[0] != 52u8) { return 12; };	// '4'
-	if (outbuf[3] != 50u8) { return 13; };	// '2'
+	let s: str = strconv.i64tos(4242i64, strconv.DEC);
+	if (s.len != 4) { return 11; };
+	if (s.ptr[0] != 52u8) { return 12; };	// '4'
+	if (s.ptr[3] != 50u8) { return 13; };	// '2'
 
 	// Probe 6 — ascii classifications (rune-taking, Hare-shaped).
 	if (!ascii.isdigit(53)) { return 14; };		// '5'