let-formatting: Insert hardline in front of the '=' instead of behind

[sjakobi]

Fixes #1301.

[sjakobi]

Note that, when there is no type annotation, the hardline still comes after the =.

[Gabriella439]

Thanks for catching this! 🙂

[sjakobi]

AppVeyor failed with a funny error:

blaze-builder-0.4.0.2: build
--  While building package blaze-builder-0.4.0.2 using:
      C:\sr\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_1.22.5.0_ghc-7.10.3.exe --builddir=.stack-work\dist\2672c1f3 build --ghc-options " -ddump-hi -ddump-to-file"
    Process exited with code: ExitFailure 1
    Logs have been written to: C:\projects\dhall-haskell\.stack-work\logs\blaze-builder-0.4.0.2.log
    Configuring blaze-builder-0.4.0.2...
    Building blaze-builder-0.4.0.2...
    Preprocessing library blaze-builder-0.4.0.2...
    C:\projects\dhall-haskell\.stack-work\install\4cd6b5f1\pkgdb\package.cache: openBinaryFile: permission denied (Permission denied)
Command exited with code 1

Let's see if it re-appears…