Pull out a generic POSIX PAL as a superclass for the Linux and generic
BSD PALs. Now we have FreeBSD and Linux adding OS-specific behaviour,
OpenBSD as a named subclass of the generic BSD PAL that doesn't add any
behaviour.
I believe a NetBSD PAL should now be identical to the OpenBSD one -
patches welcome if anyone wants to test one!
If the operating system will allocate private pages on demand for the
pagemap then use the FlatPageMap by default as it generates better code
for deallocation.
We were passing an argument less than 4K to the MAP_ALIGNED macro, which
caused an undefined shift. The compiler helpfully propagated the undef
values back to earlier in the code and gave us some exciting nonsense.
This complains if a non-const parameter is not modified. In the PALs,
the size parameter is modified only by some implementations, so we can
make it const in the ones where it isn't.
The PAL can now advertise that it supports aligned allocation. If it
does not, then the memory provider will do the alignment for it.
This change still leaves the PAL responsible for systematic testing, but
it should now be much easier to lift that out.
This does not deallocate memory until the OS tells us that we are short
on memory, then tries to decommit all of the cached chunks (except for
the first page, used for the linked lists).
Nowhere near enough testing to commit to master yet!
Move slow_allocator into a separate header in the snmalloc namespace and
rename it for consistency with the rest of the codebase. Delete its
copy and move constructors / assignment operators.
