TODO Need to incorporate end sentinal inclusion vs. exclusion.
Returns a two element array of the relationship between two Ranges.
Diagram:
Relationship Returns self |-----------| r |-----------| [0,0] self |-----------| r |---------| [-1,-1] self |---------| r |-----------| [1,1] self |-----------| r |----------| [-1,0] self |-----------| r |-----------| [-1,1] etc.
Example:
(0..3).umbrella(1..2) #=> [-1,-1]
Uses the Range#umbrella method to determine if another Range is anywhere within this Range.
(1..3).within?(0..4) #=> true