memory_pool object introduces memory contention and unnecessary
complexity. Moreover, it actually reduces performance when we compile
Lean using JEMALLOC.
Here are the numbers for corelib
jemalloc with memory_pool: 13.83 secs
jemalloc without memory_pool: 13.60 secs
@kha This commit is retracting the experiment using
`mk_fresh_name_generator_child` when creating new tasks.
The names get too long.
I'm still refactoring the code and trying to eliminate all occurrences
of `mk_fresh_name`. I still have a long way to go, but I am merging
this branch into master since it has many other fixes.
