Refactor object() to create() and autowire()

[mnapoli]

Fixes #294, see #395 for more explanations.

The main goal is to simplify DI\object() by replacing it with DI\autowire() and DI\create().

autowire() and create() will not extend the previous definition (when using multiple configuration files). That will be much clearer.

Because of that, we need to have an autowire() helper that will explicitly autowire the service (and let us override just some parameters). It has the benefit of being much more explicit.

• add DI\create() with the same signature as DI\object()
• DI\create() must not extend the previous entry with the same name
  • add test for that
• DI\create() must not go through autowiring
  • add test for that
• add DI\autowire() with the same signature as DI\object() but calls autowiring
• remove DI\object() entirely
• update the documentation
• changelog
• upgrade guide

Will fix:

• Replacing object definitions: inconsistent behavior of DI\object() #294
• Extend a previous object definition to add method calls #349

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Extract of the migration guide:

The DI\object() function helper has been removed. You should use DI\create() or DI\autowire() instead.

What should you do with your DI\object() definitions:

• if you disabled autowiring:
  • replace it with DI\create() and everything should work
• if you use autowiring:
  • replace it with DI\create() for definitions that replace autowiring completely (i.e. those that redefine all the parameters)
  • replace it with DI\autowire() for definitions that just define some parameters and let autowiring guess the rest of the parameters

If you have a single configuration file, that's it.

If you have multiple configuration files, for example if you have built a module system, then there is one thing to be aware of: DI\object() used to extend previous definitions. create() and autowire() do not extend previous definitions, they completely override them, and this is intended. The behavior of object() was confusing and hard to understand, the new helpers are more predictable and simple.