Removing the global environment from Smallstep.semantics

[jeremie-koenig]

Global environments are used as intermediate constructions when defining the step relation for individual languages, however there is no need to include them in the common interface for small-step semantics.

I have been a bit puzzled for some time as to why it's there (possibly historical reasons?), and it has been a minor inconvenience when defining operators on semantics, so I thought I'd try to remove it and submit this for your consideration.

Thanks~