This PR fixes `undefined symbol: lean::mpz::divexact(lean::mpz const&,
lean::mpz const&)` when building without `LEAN_USE_GMP`
This fixes a regression in #8089
This PR adds optimized division functions for `Int` and `Nat` when the
arguments are known to be divisible (such as when normalizing
rationals). These are backed by the gmp functions `mpz_divexact` and
`mpz_divexact_ui`. See also leanprover-community/batteries#1202.
The performance win here is pretty negligible (and of course irrelevant
with the small allocator enabled), but this is consistent with it being
used elsewhere.
Follow-up to #6598
else I see
```
[ 69%] Building CXX object runtime/CMakeFiles/leanrt.dir/platform.cpp.o
/home/jojo/build/lean/lean4/src/runtime/io.cpp:509:75: warning: 'static_assert' with no message is a C++17 extension [-Wc++17-extensions]
static_assert(sizeof(std::chrono::milliseconds::rep) <= sizeof(uint64));
^
, ""
/home/jojo/build/lean/lean4/src/runtime/io.cpp:517:74: warning: 'static_assert' with no message is a C++17 extension [-Wc++17-extensions]
static_assert(sizeof(std::chrono::nanoseconds::rep) <= sizeof(uint64));
^
, ""
2 warnings generated.
```
when building
@kha The runtime folder includes what is needed to link a
standalone Lean program. It is still contains some unnecessary files.
We will be able to remove them after we release Lean4.
