Look for prelude in the Links executables directory.

This should also fix the trailing slash issue with the LINKS_LIB environment variable.
Fix #6

Mutually-recursive types weren't being inlined properly. This is
probably a little overzealous now, as every type within a
strongly-connected component of size > is now Mu-ified, so I might
optimise this a bit later.
Merge pull request #5 from fehrenbach/sessions

Look for prelude in the Links executables directory.
Merge pull request #9 from SimonJF/rec-types-unfold

Fix #6