Compile against haskell-src-exts 1.16

[istathar]

Adjust constructors (which are full of nothing but _ anyway) so that hothasktags builds again. Since there's no test suite I can't say whether this is actually been correct, but cursory testing showed the tags file was valid.

Closes #20, maybe.

AfC

[igrep]

👍

[hesselink]

I just tested this, and it compiles with 1.16.0.1. There's still one incomplete pattern match warning though:

Preprocessing executable 'hothasktags' for hothasktags-0.3.2...
[1 of 1] Compiling Main             ( Main.hs, dist/dist-sandbox-a3afd6ca/build/hothasktags/hothasktags-tmp/Main.o )

Main.hs:37:5: Warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘extractDeclHead’:
        Patterns not matched: L.DHApp _ _ _

That's probably worth fixing if possible, otherwise people might get runtime crashes.

[istathar]

@hesselink done

[hesselink]

@afcowie I tested this, and I now have a version of hothasktags running compiled with this fix. I haven't found any problems with it, so hopefully @luqui can merge this.