fix: Merge metadata properly

[vladitasev]

• The metadata objects were not merged correctly due to calling Array.prototype.reverse several times in a row, while not considering it changes the original object, thus when merging slots, it was done in the opposite direction
• The merge was not deep so you couldn't just override a setting for a property or slot for example
• Additionally the variable klass was used only when necessary for clarity (when looping over prototypes), for the rest this is more intuitive

This changes uses the deep merge merge.js from the utils project.

closes: #454