Investigate if square size can be added as a parameter to `TreeConstructorFn`

[rootulp]

Prerequisite

1. Move nmt wrapper to rsmt2d  #248

Context

rsmt2d/tree.go

Lines 9 to 11 in 76d7440

	// TreeConstructorFn creates a fresh Tree instance to be used as the Merkle tree
	// inside of rsmt2d.
	type TreeConstructorFn = func(axis Axis, index uint) Tree

lacks a parameter for square size so multiple tree constructor functions must be registered (one per square size).

Proposal

Explore if it's possible to add a square size param to the tree constructor function.