Provide a way to set the nursery size independently from the number of capabilities

Motivation

The -A flag allows to change the nursery size. Each capability gets its own nursery, so the total amount of memory devoted to nurseries is numCapabilities * nurserySize (which is constant unless other flags like -H are used).

However, with -n, which is the default for nurseries larger than 16M, the nursery is basically shared across all capabilities.

In HLS we want to run with a nursery of at least 128M, so we set --with-rtsopts=-A128M. We also want to use all cores, so we call setNumCapabilities internally. All this is very reasonable, unfortunately it backfires quite spectacularly when running HLS in machines with >50 cores, where we end up several GB of RAM wasted due to the multiplicative effect of -A and -N.

Proposal

Change the semantics of -A to define the total space allocated to nurseries, or introduce a new flag with that effect.