Fix a bad consequence of the new story for the generic toT/fromT functions
derived from data types declarations. The problem was that they were being
generated and then discarded by the simplifier, because there was nothing
keeping them alive.

This commit
  * Adds a field tcg_keep to the TcGblEnv, which records things
    to be kept alive;

  * Makes the desugarer pin the keep-alive flag on each binding
    (it's actually a call to setIdLocalExported)

  * Removes that job from updateBinders in SimplCore


It's somewhat tiresome, but not really difficult.