rand: add methods to allow greater control over PCGSource

[DeedleFake]

For golang/go#49934.

Add two methods to PCGSource, Seed128 and State.

The standard Seed method only allows seeding with a uint64, but the PCG internal state is, effectively, a uint128. Seed128 allows the user to pass both the high and low bits for this state as two uint64s, allowing much greater control over the generator's state.

State does the opposite, returning the entire 128-bit state as two uint64s.

[DeedleFake]

Should that be Seed128() instead? It's allowing you to set 128 bits of the generator's state, not 64.

[gopherbot]

This PR (HEAD: 20a3031) has been imported to Gerrit for code review.

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[gopherbot]

Message from Rob Pike:

Patch Set 1:

(2 comments)

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[gopherbot]

This PR (HEAD: 776a471) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/exp/+/398354 to see it.

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[gopherbot]

Message from Rob Pike:

Patch Set 3: Code-Review+2

(1 comment)

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After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Rob Pike:

Patch Set 3: -Code-Review

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/398354.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR (HEAD: 7f2d908) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/exp/+/398354 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

Message from Rob Pike:

Patch Set 4: Code-Review+2

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Please don’t reply on this GitHub thread. Visit golang.org/cl/398354.
After addressing review feedback, remember to publish your drafts!