Make Url::toString(TRUE) explicit by having a Url::generate() method, as well as having GeneratedUrl::toString

Problem/Motivation

$url->toString(TRUE)->getGeneratedUrl()
Seriously, what does TRUE even mean.

Proposed resolution

• Add $url->generate()
• Deprecate \Drupal\Core\GeneratedUrl::getGeneratedUrl() in favor of \Drupal\Core\GeneratedUrl::toString().
• disputed: Deprecate \Drupal\Core\Url::toString() in favor of \Drupal\Core\GeneratedUrl::toString(). This will lead to longer constructs ($url->toString() becomes $url->generate()->toString()), but enables us to also deprecate MetadataBubblingUrlGenerator / the behind-the-scenes bubbling of metadata that has caused many LogicExceptions

Remaining tasks

User interface changes

API changes

Data model changes