Implemented CMake build system

[mtezych]

Hi J. Andrew Rogers,

I want to thank you for implementing the MetroHash library,
because it's being used extensively in the AMD Vulkan driver:

• https://github.com/GPUOpen-Drivers/AMDVLK
• https://github.com/GPUOpen-Drivers/pal

At AMD, we are working on refactoring the AMDVLK CMake build system,
during which I've implemented CMake build system for the MetroHash library.

This merge request proposes:

• Reorganization of MetroHash directory structure, isolating its interface from its implementation.

• Simple, clean and robust CMake builds system.

I want to emphasise that this change is not AMD specific.
Its goal is to improve robustness of the MetroHash library, in terms of integration it with large C++ projects.

I don't know how this change fits with your plans of introducing MetroHash v2
and what is your take on backward compatibility:

• Do you prefer gradual changes or one hard breakage?

• What kind of changes are you planning for MetroHash v2?

• When do you think MetroHash v2 will be released?

I am asking, because I want to propose changes,
which make sense in the context of your plans and to be prepared for upcoming changes.

[mtezych]

Note that, 64-bit builds have to be disabled b6c632f#diff-af3b638bc2a3e6c650974192a53c7291R78,
because in the MetroHash128crc.cpp you are using SSE4.2 intrinsic _mm_crc32_u64,
which is not available for 32-bit builds.