Add support for rewriting method signatures

[robbieaverill]

I'm not sure if this is doable yet, but assuming we start adding PHP 7 type hints etc to our existing API for SilverStripe 5, it'd be nice to be able to have the upgrader automatically rewrite methods for people's child classes. For example:

# SilverStripe 4: SiteTree
public function getCMSFields()

# SilverStripe 5: SiteTree
public function getCMSFields(): FieldList

Possible upgrader rule:

rules:
  signature_replacements:
    SilverStripe\CMS\Model\SiteTree:
      'getCMSFields()': 'getCMSFields(): FieldList'

It'd also need to import namespaces that may not exist yet, although I assume that getCMSFields(): \SilverStripe\Forms\FieldList would parse correctly when the parent's signature says getCMSFields(): FieldList with an imported namespace, so maybe that's a nice to have.

This will be important to discuss before we start implementing PHP7 changes in SilverStripe 5 for existing APIs.

[maxime-rainville]

I can see the benefit. It will probably require a bit of hair pulling to pull it off.