Use Two and Three instead of Node2 and Node3

[make-github-pseudonymous-again]

we have to decide whether we keep lazy evaluation for digit measures

[make-github-pseudonymous-again]

An intermediate step is to reuse digit measures in node measures once we have implemented #120. For instance:

node3(M, this.a, this.b, this.c);

becomes

new Node3(this.measure(M), this.a, this.b, this.c);

where this is an instance of Three.

[make-github-pseudonymous-again]

A first attempt was made at https://github.com/functional-data-structure/finger-tree/tree/perf-no-node but this seemed to decrease ops in general.