- Allow RTS options to be given using the GHCRTS environment variable.

- Fix the heap size calculation to take into account all generations.
  It's more conservative than it used to be, but now it is less likely
  that the maximum heap size will be exceeded.

- Compacting collection is turned on automatically when residency
  reaches 30% of the maximum heap size, tunable with +RTS -c<n>.
  +RTS -c turns off compaction altogether.

- The maximum heap size is off by default.  NOTE: this also means no
  compaction by default.  It is recommended that people enable a maximum
  heap size for their system using the GHCRTS environment var; eg:
  GHCRTS=-M128m.