feat: allow reuse of random source

[gammazero]

Changes to go-test module.

For tests that need to keep generating random test data, it is more efficient to continue using the same random source instead of creating a new one each time data is generated. This also applies to the generation logic itself when one part of the generation logic uses another.

This PR lets the caller create a pseudo-random number source that can be uses reused for generating test data. This is also use within the test logic itself to avoid creating unnecessary random sources.

Also included in this PR are two new functions, Name and NameSize, that generate fixed-size and random-size random name strings.

This PR fixes two minor defects in the random/files package. First, random-size names never reached the maximum size. Second, the file names were more likely to contain a "0" due to its inclusion twice in the alphabet used for file name generation.

Finally, this PR migrates from /math/rand/ to /math/rand/v2 and uses ChaCha8 as its random number source.

Summary of changes in this PR:

• Enable reuse of pseudo-random source for data generation
• Internal efficiency improvements due to reuse of random source
• Fixes minor defects with random file name generation
• Increases test coverage.
• Adds two new APIs Name and NameSize, for generating fixes and random-sized random names.
• Move from /math/rand to /math/rand/v2`

The move to a different number generator, ChaCha8, means that different values will be generated than before, and tests depending on deterministic random values need to be updated.

API Changes

• Seed value changed from uint64 to [32]byte as needed for ChaCha8
• Added Name and NameSize functions to API
• Do not support global seed
• Do not support global sequence
• Deprecated NewRand and NewSeededRand
• Make Sequence a separate type
• Added NewSeed function to create new seed of type [32]byte
• Added MakeSeed function to create [32]byte seed from uint64