Spanner: refactor Uuid to and from string implementations to obviate splitting into two uint64_ts

[scotthart]

Instead of processing the high and low 64 bits of the absl::uint128 separately as two std::uint64_t values, just use the absl::uint128 directly.

[XueSongTap]

Can I work on this？

[devbww]

Given that I already constructed to/from string functions using 128-bit values as part of the #15023 review, I would suggest that Scott assign this to me after the PR is merged.