Create an API for calculating the number bytes and shares used by compact and sparse data

[evan-forbes]

As pointed out in this comment and this comment, it will be easier to maintain and modify share encoding code if we also have a canonical API for calculating the number bytes and shares that compact and sparse share use. There are many different uses of this estimation logic, so getting a perfect API or modifying the existing code to use such an API could take a few PRs. We should track them here.

[rootulp]

We now have the following exported functions which handle the number of shares usecase:

celestia-app/pkg/shares/share_sequence.go

Lines 101 to 103 in 0b447ad

	// SparseSharesNeeded returns the number of shares needed to store a sequence of
	// length sequenceLen.
	func SparseSharesNeeded(sequenceLen uint32) (sharesNeeded int) {

celestia-app/pkg/shares/share_sequence.go

Lines 80 to 83 in 0b447ad

	// CompactSharesNeeded returns the number of compact shares needed to store a
	// sequence of length sequenceLen. The parameter sequenceLen is the number
	// of bytes of transactions or intermediate state roots in a sequence.
	func CompactSharesNeeded(sequenceLen int) (sharesNeeded int) {

I don't see an immediate need for a separate API that returns number of bytes but it would be easy to implement (number of shares needed * share size) so proposal to mark this as resolved for now? @evan-forbes