Add modern CMake build

[cdalke-havoc]

Overview

This PR adds a "modern" CMake build setup similar to some of your other projects (Seqlock, MPMCQueue) which makes it easier to consume this in downstream projects using Cmake.

Some other changes:

• Put TokenBucket under a rigtorp namespace.
• Move headers to include and test source to src
• Add the CI tests from MPMCQueue that test all platforms
• Make tests compatible with windows

The CMakeFile is copied from MPMCQueue and has test + install targets.

Test target runs the test program:

> make test
Running tests...
Test project /home/cdalke/TokenBucket/build
    Start 1: TokenBucketTest
1/1 Test #1: TokenBucketTest ..................   Passed    1.00 sec

100% tests passed, 0 tests failed out of 1

Install target copies the headers:

> make install
Consolidate compiler generated dependencies of target TokenBucketTest
[100%] Built target TokenBucketTest
Install the project...
-- Install configuration: ""
-- Installing: /home/cdalke/TokenBucket/./install/include
-- Installing: /home/cdalke/TokenBucket/./install/include/rigtorp
-- Installing: /home/cdalke/TokenBucket/./install/include/rigtorp/TokenBucket.h
-- Installing: /home/cdalke/TokenBucket/./install/lib/cmake/TokenBucket/TokenBucketConfig.cmake
-- Installing: /home/cdalke/TokenBucket/./install/lib/cmake/TokenBucket/TokenBucketConfigVersion.cmake

The tests in Debug mode for Windows and Mac seem to hang which I suspect is a tooling problem, but I just turned those off.

[cdalke-havoc]

Hi @rigtorp, thanks for making this library. This PR will make it a little easier to consume downstream via CMake's FetchContent on a few projects I'm involved with that consume this. Would appreciate your consideration!