Move EPS/HPT/etc. from HscEnv to UnitEnv

Closed Sylvain Henry requested to merge hsyl20/ghc:hsyl20/unitenv into master

These commits move unit related fields of HscEnv (EPS/HPT/etc.) into UnitEnv.

This is preliminary work to support multiple UnitEnvs active at the same time. This is needed:

Several functions will have to be refactored to use UnitEnv instead of HscEnv: this is left for future work.

Edited by Sylvain Henry

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