Deprecation helper

[amulet1]

1. Add DeprecationHelper utility class

• Emit deprecation notices pointing to the actual call site in user code
• Transparent redirection of deprecated classes to their replacements via class_alias()
• Resolve call site by walking the backtrace past autoloader frames

The helper can be used to redirect to another class by replacing the old class file with these 2 lines:

<?php
Horde\Util\DeprecationHelper::redirectClass('Horde_Array', 'Horde\Util\ArrayUtils');

Of course it would be nicer to have a single root helper class, so we could do this:

DeprecationHelper::redirectClass('OldClass', 'NewClass');

or

DeprecationHelper::redirectClass('OldClass', NewClass::class);

In addition to the actual redirecton it also calls trigger_error (or, optionally, error_log) to output deprecation notice with exact location of the deprecated code. It relies on autoloader to load old class, so only the first occurrence gets reported (overhead is small).

The debug mode output example in case of trigger_error:
[horde] PHP ERROR: The Horde_Array class is deprecated. Switch to Horde\Util\ArrayUtils (.../vendor/horde/util/lib/Horde/Variables.php:364)

And in case of error_log (if 3rd parameter is false):
DEPRECATED: The Horde_Array class is deprecated. Switch to Horde\\Util\\ArrayUtils (.../vendor/horde/util/lib/Horde/Variables.php:364)

TODO: It might be a good idea to adjust Horde's error handler to suppress backtrace output for E_USER_DEPRECATED case (the helper already reports exact location).

2. Deprecate Horde_Array class (redirect to PSR-4 implementation)

This provides a working example, but many other classes could be redirected the same way, provided their methods/parameters are 100% compatible.

TODO: Add more helper methods for other scenarios (e.g. emit custom message and pinpoint location of the caller) for deprecated method or method parameters.

[ralflang]

At least horde/util has no hard dependencies on anything notable. I will check details later.

[ralflang]

I agree with putting the helper here if you don't want a dedicated component for it.
I don't agree with log spamming library users for using a migration path we actively offer and still use in 25+ places though.

Do not use this on Horde_Array right now.