[Agda] Agda faster with GHC 7

[Dominique Devriese]

In case anyone is looking for a reason to upgrade to GHC 7: I've been
doing some (informal) benchmarks of Agda compiling Everything.agda,
and along the way I noticed the following:

ghc-6.12.1 interactive: ~1000s
ghc-6.12.1 compiled: ~950s
ghc-7.0.1 interactive: ~850s
ghc-7 compiled: ~820s

(Interactive means calling the relevant operation from ghci, compares
best to what Agda emacs mode does)

Dominique