Add repetition index to 2.1 format

[westonpace]

The repetition index is a general purpose structure that is used in the following situations:

• Miniblock encoding when there are repetition levels (in this case the index will store chunk offsets)
• Zipped encoding when there are repetition levels OR when the data is variable-width (the index will store byte offsets)
• Either miniblock or zipped encoding when there is RLE (this will be added much later)

The repetition index is not read during full scans. However, it is read during a partial scan of a page. The repetition index introduces "indirect I/O" back into the 2.1 format ( 🫠 )