Fixing type synonyms to Uniq(D)FM newtypes

[Ömer Sinan Ağacan]

Hi,

UniqFM and UniqDFM types are basically maps from Uniques to other stuff. Most of
the time we don't actually map Uniques but instead map things like Vars or
Names. For those we have types like VarEnv, NameEnv, FastStringEnv, ... which
are defined as type synonyms to UniqFM or UniqDFM, and operations are defined
like

    extendFsEnv = addToUFM
    extendNameEnv = addToUFM
    extendVarEnv = addToUFM

This causes problems when I have multiple Uniquables in scope and use the wrong
one to update an environment because the program type checks and does the wrong
thing in runtime. An example is #17667 where I did

    delVarEnv env name

where `name :: Name`. I shouldn't be able to remove a Name from a Var env but
this currently type checks.

Two solutions proposed:

- Make these env types newtypes instead of type synonyms.
- Add a phantom type parameter to UniqFM/UniqDFM.

If you could share your opinion on how to fix this I'd like to fix this soon.

Personally I prefer (1) because it looks simpler but I'd be happy with
(2) as well.

Ömer