Planning ghc-8.10.2 and cabal.port.mk switch-over

Hi Matthias and ports@ folks,

I believe now is a good time to replace ghc and cabal with the new
pieces I've been progressively sharing over the past several months.

We'll get a bunch of upgrades to the existing Haskell binary ports such
as ShellCheck and xmobar. We'll be able to easily add new packages like
pandoc should we desire. We'll reduce the maintenance burden of Haskell
library ports paying for it with lower build products reuse. This will
INCREASE the load on ports build machines on amd64 and i386. xmonad will
be even more of a special snowflake for the users when configured with a
custom xmonad.hs.

I've been running many of these new packages for months on amd64. I
built them all on i386 a couple of times.

The new version of ghc is buildable with the bootstrap threaded ghc that
kili@ kindly prepared. Cabal is still at 3.4.0.0-rc4. I estimate it will
be released mostly as it is now (allowing a wart removal from the
port). I renamed the port from cabal-install to cabal mostly to keep
port module name shorter.

I'd love to get some reviews on the tree I have in:
https://github.com/blackgnezdo/ports/commits/f7657cff4d4b5db0fe3758c4d318db32408e5773
The new piece of infrastructure (cabal.port.mk) has received some
previous attention from espie@ but no formal OK. I'd appreciate it if
somebody gave it one final look just in case something's still amiss. I
have several additional packages ported on top:
https://github.com/blackgnezdo/ports/commits/ghc810

The current state of the updated ports is attached as a tarball for ease
of reviews.

The steps I'm planning:

* Disconnect all hs-* ports from higher level Makefiles (Haskell binary
ports are broken now).
* Replace ghc, cabal, other Haskell binary ports with versions I have as
a bunch of commits in a quick succession (binary ports are good now).
* Deal with any quirks as needed.
* Wait for amd64 and i386 package snapshots.
* Fix any fallout.
* Remove hs-* ports.

Thanks
Greg