add allow_extra_fields option
feature #8133 [Form] Add allow_extra_fields option on forms (felds)

This PR was merged into the 2.5-dev branch.

Discussion
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[Form] Add allow_extra_fields option on forms

This is specially useful when creating different *nameless* GET forms on the same page (ex: pagination and search).

Commits
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c6f3dd6 add allow_extra_fields option
[Debug] Symfony debug extension
[Debug] a README for the debug extension
[VarDumper] algo to clone any PHP variable to a breadth-first queue
[VarDumper] symfony_debug ext. fast and memory efficient cloning algo
[VarDumper] add casters for per class/resource custom state extraction
[VarDumper] casters for exceptions representation
[VarDumper] casters for SPL data structures
[VarDumper] Add workaround to https://bugs.php.net/65967
[VarDumper] casters for PDO related objects
[VarDumper] casters for removing the entity manager from Doctrine proxies
[VarDumper] interface for dumping collected variables
[VarDumper] CLI dedicated dumper and related abstract
[VarDumper] HTML variant of the CLI dumper
[VarDumper] spec for json based representation of PHP vars
[VarDumper] json dumper for interoperable displaying
[VarDumper] tests for JsonDumper
[VarDumper] tests for CliDumper
[VarDumper] tests for HtmlDumper