Reuse previous compile results for bcr files

[phile314]

Currently, bcr files are always compiled from scratch. It would be good if this were avoided when the
bcr files have not changed. UHC already does this for Haskell files (.hs).

This requires some larger changes in the UHC Compiler Driver, and first requires a refactoring of the UHC compiler driver.

It is unlikely that this change makes it into UHC in the near future.