Export `subtract` in the Prelude
Initialize undefined elements to show their index
Switch from test-framework to tasty
Relax syntactic constraint
Fix loop optimization and add testcase
Update travis config and constrain stm when installing tasty
Remove unneeded 'cabal update' from travis config
Do not export scan from Feldspar.Vector

It is in Feldspar.Vector.Internal
Make scan return the initial state as the first element (consistent with Prelude.scanl)
Relax types of  and
Revert "Make scan return the initial state as the first element (consistent with Prelude.scanl)"

This reverts commit ad9267564ad03680ce3320c61040e7c442516893.