Fix build breaks in Paka, HamletAst and SockeyeInstantiator

[satnam6502]

When attempting to execute the command:

$ ../hake/hake.sh -s ../ -a x86_64

from the install instructions I got several build breaks due to the Haskell code no longer working with a recent version of GHC. I made changes to hide the Prelude version of <> and also to change a pattern match to avoid falling foul of the new automatic pattern match failure handing (see transitional monad we could try to use instead which would be better than the evasive approach I take in this patch).

Developer's Certificate of Origin 1.1

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        have the right to submit it under the open source license
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    (b) The contribution is based upon previous work that, to the best
        of my knowledge, is covered under an appropriate open source
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    (c) The contribution was provided directly to me by some other
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    Signed-off-by: Satnam Singh <[email protected]>

[barrelfish-bot]

Can one of the admins verify this patch?