support ghc 8.4 & latest stackage nightly

[simonmichael]

ghc 8.4 was released last week, and is now used by stackage nightly. We have been dropped from stackage nightly, presumably because of ghc 8.4 build or install plan issues. Fix/monitor until we are fully back in the latest stackage nightlies.

[simonmichael]

stack complains about persistent-sqlite not found commercialhaskell/stack#3923

[simonmichael]

https://www.reddit.com/r/haskell/comments/842e2k/stackage_nightly20180313_with_ghc_841_is_out/dvqvnpe/

[simonmichael]

"shadowed dependencies" build problems with ghc 8.2.2, related issues. Workaround required: nuke ~/.stack entirely.

[simonmichael]

Track progress on unblocking packages in build-constraints.yaml: https://github.com/fpco/stackage/commits/master/build-constraints.yaml

[simonmichael]

Blocked by: gregorycollins/hashtables#45

[simonmichael]

hledger-api blocked by: zmthy/http-media#25

[simonmichael]

hledger-web blocked by yesodweb/wai#686

[simonmichael]

hledger-lib test suite blocked by joelburget/easytest#4

[simonmichael]

Building with -Werror hampered by commercialhaskell/stack#3918

[simonmichael]

hledger-ui on linux blocked by
kolmodin/hinotify#25
commercialhaskell/stackage#3501
haskell-fswatch/hfsnotify#75
haskell-fswatch/hfsnotify#77
commercialhaskell/stackage#3548

[simonmichael]

hledger-lib doctests with ghc 8.4 on mac blocked by sol/doctest#199, more discussion at haskell/cabal#5222. This shouldn't affect stackage though.

[simonmichael]

hledger-web blocked by json commercialhaskell/stackage#3502
commercialhaskell/stackage#3504

[simonmichael]

hledger-api blocked by swagger2 commercialhaskell/stackage#3505

[simonmichael]

All main hledger packages are back in stackage nightly at last. \o/