Discriminate json_number's three numeric slots into one union

json_number used to carry three parallel 8-byte fields (a double plus
both a 64-bit unsigned and a 64-bit signed slot for the large-integer
cases) even though at most one of the integer slots is ever the
canonical value for any given number. Collapse them into a
discriminated union:

    enum repr_t { REPR_DOUBLE, REPR_LARGE_UNSIGNED, REPR_LARGE_SIGNED };
    repr_t repr;
    union { double d; unsigned long long u; long long s; } value;

Callers keep the same read API: number() returns the appropriate
double, large_unsigned() returns the ull (or 0 if not currently stored
that way), large_signed() likewise. Writes go through new set_number /
set_large_unsigned / set_large_signed methods that keep the
discriminator and the union value in sync.

This was prompted by an observation that moving json_type to the end
of the object should shrink things via tail-padding reuse. Empirically
the type-at-end rearrangement saves nothing on its own (every
subclass payload is 8-byte aligned so it can't slot into the 4-byte
tail), but the discriminated-number redesign hits the same idea from
a different direction: adding the 4-byte `repr` to json_number makes
the class non-standard-layout, which lets the Itanium ABI pack `repr`
into the base's 4-byte tail padding at offset 20. The union value
then starts at the natural offset 24, and json_number ends at offset
32 -- a 33% reduction.

Per-node sizes:
  json_object (TRUE/FALSE/NULL)  24 bytes
  json_number                    32 bytes  (was 48)
  json_string                    48 bytes
  json_array                     48 bytes
  json_hash                      48 bytes

Numbers dominate real GeoJSON (every coordinate is one), so the net
memory win on a typical parse is substantial.

Co-authored-by: Cursor <cursoragent@cursor.com>

jsontool.cpp

@@ -326,7 +326,7 @@ void join_csv(json_object_ptr j) {
 					vo = s;
 				} else if (attr_type == JSON_NUMBER) {
 					auto n = std::make_shared<json_number>(properties.get(), properties->parser);
-					n->number_value = atof(v.c_str());
+					n->set_number(atof(v.c_str()));
 					vo = n;
 				} else {
 					abort();