Custom Monoid instance for Utf8Builder

[lehins]

Because of derived instance with GeneralizedNewtypeDeriving, mconcat breaks list fusion and causes extra allocations when used with Utf8Builder. Here is a way to reproduce:

  runSimpleApp $ do
    logInfo $ foldr (<>) mempty $ fuseThis ["foo", "bar", "-", "<>"]
    logInfo $ foldr mappend mempty $ fuseThis ["foo", "bar", "-", "mappend"]
    logInfo $ mconcat $ fuseThis ["foo", "bar", "-", "mconcat"]

fuseThis comes from Data.List.Fusion.Probe

Code above results in:

foobar-<>
foobar-mappend
fuse-builder-exe: fuseThis: List did not fuse
CallStack (from HasCallStack):
  error, called at ./Data/List/Fusion/Probe.hs:52:16

But with the fix in this PR:

foobar-<>
foobar-mappend
foobar-mconcat