This commit makes two main changes:

1. Various configuration statistics that were duplicated in the
   GenerationConfig type have been removed, in favour of using the
   source values from the Config type directly.

2. Updating garbage collection statistics has been reworked, to prevent
   garbage collection from running too frequently.

The first change results in processes requiring less memory, though we
are talking about only a handful of bytes.

The second change is more interesting. Prior to this commit, the number
of allocated blocks was reset to the number of live blocks. If this
number would be great enough, the next garbage collection cycle could be
scheduled much sooner than necessary.

To illustrate this, let's say our threshold is 10 Immix blocks. For the
first garbage collection to occur, we thus need to allocate 10 Immix
blocks. If 5 Immix blocks remain live after a cycle, we would only need
to allocate 5 Immix blocks to trigger the next cycle; instead of
allocating 10 Immix blocks. This commit fixes this by resetting the
number of allocated blocks after every cycle.