Merge instant functionality into DOLFIN (?)

[Anders Logg (Chalmers)]

I suggest merging (reimplementing) the functionality of Instant for JIT compilation of Expressions into DOLFIN to reduce the number of repositories / projects.

[Martin Sandve Alnæs]

Just let instant be, it's used outside of fenics, and copy whatever you want to dolfin. Probably better to share most functionality with dijitso though, less testing and maintenance work that way. I suggest mapping out what exactly dolfin needs from instant that dijitso doesn't provide (I guess mainly swig and cmake support) before deciding on the approach.

[Martin Sandve Alnæs]

Although I question the cost-benefit ratio here. Installing instant has never been a bottleneck, and it's not a maintenance burden as it changes rarely now.