[ fix ] added missing constructorForm in LHS.Unify
[ sharing ] resurrected pointer term representation

Type checking times are more or less the same,
slightly slower type checking (of std lib), but faster
serialisation.

Big win is actual call-by-need compile-time evaluation.
[ sharing ] updated error messages and interaction output

Things get more evaluated now, since evaluation gets shared.
[ sharing ] added more missing ignoreSharing
[ sharing ] updated error messages again
[ sharing ] enable forcing pointer equality for terms that compare equal
[ sharing ] debug printing of the entire signature
[ sharing ] check pointer equality before syntactic equality
[ sharing ] disable making equal terms pointer equal

I'm not sure we can trust it to not create loops.
[ sharing ] more normalisation in error message
[ sharing ] test/fail error message