Improve evar_map handling in tmMkDefinition and friends

[MathisBD]

Fixes (part of) #1108.

This PR fixes a bug in the implementation of the template monad. When modifying the global environment (tmDefinitionRed and friends), we currently ditch the old evar map and create a fresh one using Evd.from_env (see e.g. template-coq/run_template_monad.ml, in the main loop, the TmDefinition case).

This is a problem since the evar map can contain evars and universes which are still used afterwards, and we discard them, leading to Undeclared universe errors e.g. when calling tmMkDefinition multiple times.

The fix is to replace :

let evm = Evd.from_env (Global.env ())

by :

let evm = Evd.update_sigma_univs (Environ.universes env) evm

in the relevant places.

[MathisBD]

I tried manually building MetaCoq and the only error seems to be in translations/standard_model.v.