(svex-envs-similar x y) is like alist equivalence for svex-envs: environments are similar if they bind all variables to the
same values, in the sense of svex-env-lookup.
(svex-envs-equivalent x y) is a stronger form of alist
equivalence for svex-envs than svex-envs-similar: environments
are similar if they bind all variables to the same values, in the sense
of svex-env-lookup, and they bind the same variables.
(svex-env-<<= x y) checks whether an entire svex-env
conservatively approximates another: i.e., is every variable's value in x
an approximation of its value in y?
