MerkleTree, Segment, ColMatrix changes for GPU RPO support

[andrewmilson]

Changes:

• Make MerkleTree fields public so the merkle nodes can be generated in a custom way (on the GPU in my case)
• Add merge_column to ColMatrix to append columns
• Make get_offsets public
• Make Segment public
• remove .data() from Segment and instead implement Deref (gives access to the data Vec instead of the slice which I need to call .capacity() on)

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[irakliyk]

Looks great! Thank you! I left a few small comments inline - mostly related to API ergonomics.

[irakliyk]

All looks good! Thank you!