This PR ensures that `Nat`s in `.olean` files use a deterministic
serialization in the case where `LEAN_USE_GMP` is not set.
This is a simplified version of
https://github.com/leanprover/lean4/pull/2908.
Thanks to `mmap`, startup time is not necessarily related to this
figure, but it can be used as a rough measure for that and how much data
the module depends on, i.e. the rebuild chance.
Also adds new cumulative benchmarks for this metric as well as the
number of imported constants and env ext entries.
When used with `COMPRESSED_OBJECT_HEADER`, Lean uses a compressed
object header where only 32-bits are reserved for the RC.
The motivation is performance, in our experiments, it is faster to
access a 32-bit counter than a 45-bit one.
With a smaller RC, we can use 8-bits for the memory kind information,
and speedup its access.
