Generalize single-assignment variables.

Generalize the single-assignment variables to a lattice type --
derflow_ivar, which is an immutable variable.  Remove any specific
handling for the undefined type, and compute merges over the immutable
type.  Convert the normal read to a blocking threshold read on the
strict inflation from undefined.
Rename derflow.

Rename derflow to lasp.
Fix eqc.

Fix eqc test after the introduction of the new single-assignment
variable lattice.