Use std::allocator_traits::rebind_alloc instead of std::allocator::rebind

[vlstill]

The latter is deprecated in C++17 and removed in C++20.

[vlstill]

The is the first part of #4297 which is too large to be done in one go.

I am fully aware this is the same change as #4364, but since that one does not pass and I had this patch already done, I've raised this separate PR.

[smolkaj]

Sweet, thanks!

[ChrisDodd]

Do we need an #if __cplusplus >= 20170101 ... here as rebind_alloc does not exist in earlier versions?

[fruffy]

Do we need an #if __cplusplus >= 20170101 ... here as rebind_alloc does not exist in earlier versions?

Both of P4C's build tools (CMake and Bazel) require C++17 and the corresponding compilers as minimum version. It was introduced with this PR: #3547. Over the past year no one has raised any concerns.