Add return types and update PHPDoc bool types

[RobinvanderVliet]

• Add return types to most functions.
• Change "boolean" to "bool" in PHPDoc for consistency, instead of using both mixed.

[cweiske]

Remove return type from createInstance because of unit test

The unit test could be adjusted.

I'm wary because adding return types to methods will be a breaking change - classes extending JsonMapper will not have those return types in their overridden methods, and PHP will complain about that:

Declaration of MyChild::getString() must be compatible with MyParent::getString(): string

So this would require a new major version (6). In that case the unit test needs to be adjusted, so that we have proper return types for all methods.