Psalm refactoring

[danog]

Various psalm-related fixes:

• Turn all @psalm-param and @psalm-return annotations into @param and @return annotations (this avoids issues where @psalm- annotations are treated as second-class and forgotten about, and thus proper types aren't even mentioned in some cases). All major IDEs already support Psalm syntax in @param and @return annotations.
• Autogenerate and run static analysis tests for both return and non-return usages
• Remove some invalid assertions, and move assertions that were invalidly put into @psalm-param into a proper @psalm-assertion
• Fix native return types of the mixin
• Add a Psalm plugin to provide properly asserted return types to all methods
• Describe some return assertions using templates (as fallback if the plugin isn't installed)
• Fix template-based assertions in the mixin

[danog]

Ping @shadowhand :)

[danog]

Rebased!

[shadowhand]

Overall, looks good. The changes to composer.json are misplaced, though.

[danog]

Fixed!

[shadowhand]

Released in v2.4.0.

[danog]

Thanks!