Abstract over getrusage

[Ericson2314]

EvalState::printStatistics uses getrusage, which is not portable to Windows. https://github.com/postgres/postgres/blob/master/src/port/win32getrusage.c is a compat shim however from venerable PostgreSQL that I think is appropriately licensed for us.

We should create a new interface in libutil for what EvalState::printStatistics needs. (It doesn't need to have the same type signature as getrusage if that helps) and then implement Windows and Unix implementations.

Originally posted by @Ericson2314 in #8901 (comment)

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-on-windows/1113/109