* Use MaybeUninit in DecInt

* Fix DecInt::new panicking on integers greater than 64 bits and optimize out any panics in DecInt::new

* Add tests so people messing with DecInt can catch UB with miri

* Use new itoa

---------

Co-authored-by: Alex Saveau <[email protected]>

Co-authored-by: Alex Saveau <[email protected]>

Linux added fields to `xdp_umem_reg`, so disable rustix's sizeof check
for now, to unbreak the `test_sizes` test.

The code is compatible with version 0.52.0 and version 0.59.0.

Additionally, bump errno to 0.3.10 to get this requirement on
errno as well.

Remaining two failing tests need more investigation:
    unix::test_unix_msg_with_combo
    unix_alloc::test_unix_msg_with_combo

This is safe since all methods that mutate the internal state require
a &mut self. It also makes behavior consistent with the linux_raw
backend (whose `fs::Dir` is Sync+Send).

Fixes: #1230.

Even when Solaris libc doesn't define LOCK_SH, LOCK_EX, LOCK_NB and LOCK_UN (which are flock() related).