Refactor to use C#7 features

[apkd]

This pull request introduces a number of non-functional changes that are intended to make code more concise and readable. These include:

• Usage of expression-bodied members in short methods and getters (a lot of those!)
• Usage of pattern matching to simplify type tests
• Usage of local functions

All these dropped braces sometimes amount to a 20%-30% decrease in line count per file. Obviously these are stylistic changes so it's up to your opinion whether merging these changes represents an improvement.

[joashc]

Thanks for this, this makes things a lot cleaner.

Getting some merge conflicts here after merging #6, if you fix those I'll get this merged.