Implementation of a data structure for composable text ranges.
A Rhizome consists of an origin and destination Interval, where Intervals
have a basis, a start integer, and an end integer. The basis is any type with
a string id. A Rhizomes object holds two lists of rhizome objects---LinkRhizomes and SourceRhizomes.
In this project a monoid is defined on the type Rhizomes, under the method
Rhizomes#compose.