Failure propagation is crucial for splitter and component steps
because that's the only signal for them to figure out if they
could be completely solved or have to fail.
For example, if one of the component steps created by "split"
fails it would cascade to the rest of the pending component steps
receiving "fail" signal and ultimately result in failure of the
"split" when it's re-taken.
* `SplitterStep` is responsible for running connected components
algorithm to determine how many independent sub-systems there are.
Once that's done it would create one `ComponentStep` per such
sub-system, and move to try to solve each and then merge partial
solutions produced by components into complete solution(s).
* `ComponentStep` represents a set of type variables and related
constraints which could be solved independently. It's further
simplified into "binding" steps which attempt type variable and
disjunction choices.
* "Binding" steps such as `TypeVariableStep` and `DisjunctionStep`
are responsible for trying type binding choices in attempt to
simplify the system and produce a solution. After attempting each
choice they introduce a new "split" step to compute more work.
The idea so to split solving into non-recursive steps,
represented by `SolverStep`, each of the steps is resposible
for a unit of work e.g. attempting type variable or
disjunction bindings/choices.
Each step could produce more work via "follow-up" steps,
complete "partial" solution when it's done, or error which
terminates solver loop.
