Swapped arguments $groups and $subPath in ExecutionContext::validate and ExecutionContext::validateValue, so they match their interface signatures again.
With this refactoring comes a decoupling of the validator from the structure of
the underlying metadata. This way it is possible for Drupal to use the validator
for validating their Entity API by using their own metadata layer, which is not
modeled as classes and properties/getter methods.
A new ExecutionContext is now created everytime that GraphWalker::walkConstraint() is
launched. Because of this, a validator B launched from within a validator A can't break
A anymore by changing the context.
Because we have a new ExecutionContext for every constraint validation, there is no point
in modifying its state anymore. Because of this it is now immutable.