Convert enum objects into scalar values during form submission

[yann-eugone]

When enum values are objects, the submitted data is the position of the value in the "choices" option.
It works but do not seems to be a good default value:

• when working on a "MyCLabs enum", we want to submit the enum value
• when working on a "PHP native enum", we want to submit the enum case name
• when working on a "PHP native backed enum", we want to submit the enum case value

In order to avoid BC, the "enum_choice_value" option is introduced, and a deprecations is raised when it is not configured explicitely (forcing developer to think about the desired behavior).
But in the 5.x, the default behavior will be inverted, and the option will be true by default.

[J-Ben87]

This might require a bit of documentation 🙂