Add ConstantScanner to Zend\Code\Scanner

[MatyCZ]

Fix bug introduced in #4457 and add some new features.

• Can parse constants of type float, number or string
• Can parse constants with docblock on previous line
• Can parse constants with self:: reference
• Added has* methods for constant and property
• Updated docblocks

There is one BC break. Method getConstants() don't return constant names now, but return instance of ConstantScanner.

[weierophinney]

This is definitely a BC break, and we can't do that.

I'd argue instead for the following:

• have an optional flag argument to getConstants(), $namesOnly, that defaults to true.
• When $namesOnly is true, it will proxy to getConstantNames(). Additionally, raise an E_USER_DEPRECATED error, so that developers know they should alter their code to use getConstantNames() if they want the original behavior.
• when the flag is false, do the new behavior.

This will allow developers to opt-in to the new behavior, and give us a migration path.

[MatyCZ]

@weierophinney Addition of E_USER_DEPRECATED breaks tests. It's OK or I must change it to E_USER_NOTICE with same message?

[weierophinney]

@MatyCZ alter the tests to check for the error as well as the behavior.