Reorganize header files and unit tests.

[lambdafu]

This moves around the source files a bit. Header files and unit tests are now colocated to the source file. This makes it easier to edit them at the same time. Also, header files are installed in a flat namespace and a single directory. Hopefully this removes some mental strain from remembering the hierarchy and the namespaces.

[codecov[bot]]

Codecov Report

Merging #52 into master will decrease coverage by 68.72%. The diff coverage is 98.41%.

@@             Coverage Diff             @@
##           master      #52       +/-   ##
===========================================
- Coverage   85.33%   16.61%   -68.73%     
===========================================
  Files          45      179      +134     
  Lines         648    30756    +30108     
===========================================
+ Hits          553     5111     +4558     
- Misses         95    25645    +25550