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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    (a) The contribution was created in whole or in part by me and I
        have the right to submit it under the open source license
        indicated in the file; or

    (b) The contribution is based upon previous work that, to the best
        of my knowledge, is covered under an appropriate open source
        license and I have the right under that license to submit that
        work with modifications, whether created in whole or in part
        by me, under the same open source license (unless I am
        permitted to submit under a different license), as indicated
        in the file; or

    (c) The contribution was provided directly to me by some other
        person who certified (a), (b) or (c) and I have not modified
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    (d) I understand and agree that this project and the contribution
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        this project or the open source license(s) involved.

    Signed-off-by: Satnam Singh <satnam@google.com>

[barrelfish-bot]

Can one of the admins verify this patch?