compat: std::pmr -> compat, until moved from experimental
compat: std::pmr -> compat, until moved from experimental

After discussion[0], it was decided seatar would use
std::experimental::pmr::{memory_resource,polymorphic_allocator} even
though they're currently experimental, and aren't supported in C++14.

In order to reduce future work when GCC moves them from
std::experimental back to std, we'll create an alias for them in
std-compat.hh.

[0] https://groups.google.com/d/msg/seastar-dev/G2vI5dvc8Qg/_C8o0p-eAQAJ

Signed-off-by: Elazar Leibovich <f66bf065cb40f105b126445a801a635b6b1cbf14@gmail.com>

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