Going to change the default Haskell compiler to GHC 9.0.1

[PHO <pho%cielonegro.org@localhost>]

Hi,

I'm going to change the default Haskell compiler to lang/ghc90 (GHC
9.0.1) within a few days unless there are any objections. All of the
Haskell packages have been confirmed to be working, or at least
compiling with the new compiler, except:

* devel/hs-ghc-mtl no longer compiles so it'll be marked as BROKEN. It
depends on the GHC internal API and couldn't survive the compiler
update. No packages depend on it luckily.

* devel/hdevtools has been marked as BROKEN for the same reason since
GHC 8.8 (the current default). The upstream is seeking for maintainance
takeover, but no one has shown up so far.

* devel/hs-exceptions is now part of GHC so it's going to be removed
from pkgsrc.

Bootkit for the new compiler is available on the following platforms:

* Darwin/amd64
* FreeBSD/i386
* FreeBSD/amd64
* NetBSD/amd64
* SunOS/amd64 (built on OmniOS. I hope it works on SmartOS as well)