Standardise file naming, header guards

[willgittoes-dd]

Standardises source file naming on TitleCase.ext, which is more windowsy than the alternative snake_case.ext.

Same deal with header guards. There was a mix of #defines and #pragma onces, and I settled on the latter as being more common in windows code.

[lucaspimentel]

@willgittoes-dd: You and @dd-caleb should agree on one style for filenames and #pragma once. Looks like you both are moving things in different directions. Caleb has been renaming file from CamelCase to snake_case, recently, and changing #pragma once to #ifdef. Let's just pick one style guide (Google C++ Style Guide?) and stick to it.

And don't worry about the C++ code being more "Windows-y." It will need to compile on Linux at some point, too.

[dd-caleb]

Yeah I had been removing the pragmas and renaming the files to match the google style guide. Don't have a strong opinion one way or the other, but if we use pragma once we should make sure it works in non-windows.

[willgittoes-dd]

ehehe, I'm happy to flip this to doing it all the other way :) I picked these conventions because they were the ones used in the ClrSamples repo, but I don't mind either way.

I'll make a version of this that uses the opposite conventions (as per Google C++ style guide), and you can approve whichever :D

[willgittoes-dd]

eyyyy, try this one instead: #103

[lucaspimentel]

We're going with #103 instead. Thanks!