Extract AST to its own library

[thekid]

This pull request extracts Node and Value classes into https://github.com/xp-framework/ast as a prerequisite for xp-framework/rfc#327

API changes

The API has been changed to be more concise in naming and more consistent and intuitive in parameter order.

Types:

• ClassValue is now ClassDeclaration
• InterfaceValue is now InterfaceDeclaration
• TraitValue is now TraitDeclaration

new ClassDeclaration($modifiers, $name, $parent, $interfaces, $body[, $annotations[, $comment]])
new InterfaceDeclaration($modifiers, $name, $parents, $body[, $annotations[, $comment]])
new TraitDeclaration($modifiers, $name,  $body[, $annotations[, $comment]])

Type members:

• MethodValue is now Method
• PropertyValue is now Property
• ConstValue is now Constant

new Method($modifiers, $name, $signature[, $body[, $annotations[, $comment]]])
new Property($modifiers, $name, $type[, $expression[, $annotations[, $comment]]])
new Constant($modifiers, $name, $expression)

Related:

new Signature([$parameters[, $return]]) 
new Parameter($name, $type[, $default[, $reference[, $variadic[, $promote[, $annotations]]]]])

Others

All other node values are now named ...Statement or ...Expression