There are many intentional uses of the c99 designator initialization
syntax in the codebase.

This is necessary for the macOS alloc code to compile.
The change in error_analysis_test.cpp is necessary because a
non-Principia function is called which returns a ::std::vector.
I'm not sure why the friend std::pair in discrete_trajectory.cpp
needs full qualification.

Specifically, force include magic/malloc_allocator.hpp on macOS
builds via clang argument.

Split principia::std into a separate file.
Disabled principia::std for non-macOS builds.

Use system allocator for containers on macOS (bypassing Unity's allocator)