Quick hack to work around the dynamic exception crashes in GHCi.  The
problem is this: the Data.Typeable library (Data.Dymamic in previous
versions of GHC) maintains an internal cache mapping type names to
unique numbers, and this cache is used for fast comparisons on
TypeReps.  In GHCi, there are actually two versions of the
Data.Typeable library loaded: one linked to GHCi itself, and the other
dynamically loaded, so there are two copies of the hash tables.

The problem is that if we have a Dynamic value generated using one set
of hash tables, it will erroneously appear to be of a different type
when the other hash tables are used.

The hack I've instigated is to use the central RTS genSym (which
already exists) to generate the unique Ids, so that the two copies of
the dynamic library will be using distinct Ids, and Dynamics from one
will never be recognisable to the other.