Move A* code to cpp file

[ranisalt]

Pull Request Prelude

• [x] I have followed proper The Forgotten Server code styling.
• [x] I have read and understood the contribution guidelines before making this PR.
• [x] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

In order to allow the compiler to further optimize A* code, move classes and as much as possible of the code to the cpp file so it can be aggressively inlined and omitted. While at that, add const to values that aren't mutated and modernize code.