Workaround cores inability to use "new in initialisers" with Pinto objects

Any custom code using pinto.internal.hook_theme must also follow our lead, by unserialising. 🤮

There shouldn't be any runtime performance penalty, as the result of \pinto_theme is subsequently cached elsewhere on first-use.

New minor as its a soft break to internal functionality.

Notably, our private "common" library uses it via setPintoHookTheme, so will need 0.5.1, and updates to code:

- param type array -> string
- add unserialize before passing to array_keys

Notably I could tear apart the hook theme regeneration stuff in compiler pass and move it to a service, but Id still need to construct hook_theme default variables by looking thru definitions, then doing reflection stuff. Which is a PITA.

...or nullify all default theme variables

Merged this simple workaround.