Eliminate the post-zygote heap and reuse the zygote allocation heap

Eliminate the post-zygote heap and reuse the zygote allocation heap
for application allocations.  Previously, applications were given
their own heap separate from the zygote.  However, the zygote never
allocates more than 10s of objects most of which quickly become
garbage.  After an application fork, these objects are reclaimed,
dirtying the pages they and their malloc structures reside on.  This
is a further win for the GC as it results in one fewer mspace to
considered for range checks and bitmap traversals.