Currently bindings where inferred on every `bindTypeVariable` call,
but that's wasteful because not all binds are always correct. To
avoid unnecessary inference traffic let's wait until re-activated
constraints are simplified and notify binding inference about new
fixed type only if all of them are successful.
Associate potential bindings with a constraint graph node and
start keeping track of added and removed constraints.
This is not a complete implementation because the ability to
infer bindings transitively is missing.
Binding inference requires constraint propagation through to referenced
type variables, in current "fixed bindings" storage mode it is impossible
to figure out whether tracked type variables are referenced by or from
a type variable.
Splitting storage for referenced by/from also helps to provide more
meaningful debug output and make sure there are no attempts to insert
duplicate references.
