Support unsigned 64-bit integers (uint64_t) exceeding INT64_MAX (#501)

[sndth]

Large positive decimal scalars whose value exceeds INT64_MAX (e.g. xxHash digests, high-half UUID fields) were silently parsed as STRING nodes instead of INTEGER nodes. Calling as_int() on such a node then threw type_error, making the value unreachable without ugly string post-processing.

This PR adds transparent uint64_t storage support inside the existing int64_t field via a new style flag, uint_bit, and exposes two new public APIs: is_uint() and as_uint().

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