Minor (style) optimizations

[tbe]

Just a few optimizations that should make the code more readable and increase the performance in some cases:

• replaced string literals with raw string literals where quoting was required before
• made members static/const where possible
• used override instead of virtual for derived objects
• replaced a bunch of implicit copies of objects with const &
• replaced size() with empty() where possible
• simplified some boolean expressions and loops
• replaced 0 with nullptr for ptr types

and some other very minor changes.

I grouped the changes by their nature in commits, so cherry picking should be easy.

Details, why i think these changes are nice are in the individual commit messages.

[tbe]

I close this PR in favor of a major cleanup and rework of the codebase