Make clearNursery free

Summary:
clearNursery resets all the bd->free pointers of nursery blocks to
make the blocks empty.  In profiles we've seen clearNursery taking
significant amounts of time particularly with large -N and -A values.

This patch moves the work of clearNursery to the point at which we
actually need the new block, thereby introducing an invariant that
blocks to the right of the CurrentNursery pointer still need their
bd->free pointer reset.  This should make things faster overall,
because we don't need to clear blocks that we don't use.

Test Plan: validate

Reviewers: AndreasVoellmy, ezyang, austin

Subscribers: thomie, carter, ezyang, simonmar

Differential Revision: https://phabricator.haskell.org/D318
Add +RTS -n<size>: divide the nursery into chunks

See the documentation for details.
Document +RTS -xq
Make the linker API thread-safe

We used to be able to rely on the client to use the API in a
single-threaded way, but now that the GC calls into the linker to
unload objects this isn't a safe assumption.
Add purgeObj() to remove the symbol table entries for an object

This allows us to replace an object without actually unloading the old
object, which is necessary when we know we have references to the old
object so it can't be completely unloaded.  Using unloadObj() would
cause the GC (CheckUnload) to repeatedly and fruitlessly try to unload
the old object.