Add Farm Hash conditioned upon `nimPreviewHashFarm` as 64-bit `Hash` (#23735)

Unlike present Nim this actually fills `Hash` for `string` & related.

For the curious, note that `hashData` remains the aboriginal Nim string
hasher & `import hashes {.all.}` allows simultaneous test/time of {orig,
murmur, farm} on your favorite CPU & back end compiler.

Update tests also conditioned upon `nimPreviewHashFarm` so they should
pass either with or without that `define` on.

In `--jsbigint=on` mode, only the lower 32-bits of `Hash` match nimvm &
run-time values because `type Hash = int` and on JS int=int32, not int64
as for 64-bit Nim platforms. Due to the matching, `const` Table should
match run-time `Table` on all platforms.

To operate in `--jsbigint=off` mode is feasible but needs much "double
precision mul/xor/ror/shr-arithmetic"-style work. That is distracting &
also of questionable value since JS added BigInt in 2018, ringabout
added Nim support for it in 2021 & `nimPreviewHashFarm` is unlikely to
swap from an opt-in to an opt-out default before 2025..2026 which will
have given a backward looking time window of 7..8 years for deployment
platforms - reasonably generous.

Add a changelog entry for 2.2.

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