Add complicated computation of v ~_E w to Execution

Add complicated computation of v ~_E w to Execution

The relation `v ~_E w` is used when traversing
a WakeupTree to determine is an "equivalent"
execution is already noted in the tree. This commit
adds the first "working" implementation of
several components to this process, viz.

1. Assuming that iteration has been implemented
(which is to come in future commits), the WakeupTree
now implements the `insert_E(v, B)` operation, where
`B` refers to the tree instance itself.

2. The `Execution` class can now determine what
is needed to complete the `insert()` operation,
specifically determining first if a) two sequences
`v` and `w` are related by `v ~_E w` and b)
what the smallest sequence `w'` is such that
`w [= v.w'`. The latter computation uses the neat
observation that (w / v) is the smallest such
sequence (viz. removing anything in `w` that's in
`v` "one at a time") and computes this value as
it is in the process of determining whether the
`~_E` relation noted holds