Use a single name for each variable that references square size

[evan-forbes]

This repo uses three names for the same thing: origSquareSize, squareSize, k. Is it safe to assume they are all interchangeable? If so, would it aid readability to standardize on one? My impression is that readers who are familiar with https://celestiaorg.github.io/celestia-specs/latest/rationale/message_block_layout.html#non-interactive-default-rules are able to infer k == squareSize so this may not be confusing for other readers but may be helpful for new readers who aren't as familiar with the specs.

Originally posted by @rootulp in #680 (comment)

We should use a single reference to square size for the entire repo.

[rootulp]

If we opt to use SquareSize and squareSize, this implies a breaking change to protobuf types: https://github.com/rootulp/celestia-core/blob/114375ab4f4e2976ff3f6dde1caeddb430a14f4e/types/block.go#L1034

[rootulp]

I found another instance in

celestia-app/proto/payment/tx.proto

Line 34 in 27632dd

uint64 k = 1;