Figure out intrinsics for hysteresis

[Keno]

The following was proposed:

p = finite_state(Bool)
p[]::Bool
event!(cond, dir=:both) do
     transition!(p, !p[])
end

[Keno]

Thinking about this some more, I don't think I like the callback versions, so maybe:

p = discvar(Bool)
transition!(cond, :both, p, !p)

[Keno]

Final design. Keep discvar (renamed discrete) and add:

"""
	ett!(on_change, assign, val)

Declare an edge-triggered transision. When `on_change` (in the discrete domain)
changes value (under egality), the discrete variable `assign` is set to `val`.
The change to `assign` may trigger further edge-triggered transitions. It is
an error for multiple transitions to trigger simultaneously, unless one such
transition strictly dominates the other.

Note: `assign` must be a bare discrete variable (unlike ordinary continuous equations
where the assignment is inferred or non-linearly solved).
"""
ett!

"""
	pre(x)

When evalutated in the continuous domain, this intrinsic is equivalent to
the identity function. When evaluated at event triggers, this function returns
the value of `x` using the left-limit of all discrete and continuous algebraic
variables.
"""
pre

"""
	threshold(x)

Detects zero-crossing of the continuous-domain expression `x`. Returns `true`
when `x` is about to be `> 0` and false otherwise, generating an edge-trigger
event at the crossing. Note that the actual value of `x` at the event-trigger
may have a small difference from `0` (in either direction) according to the
solver tolerance.
"""
threshold