Making uniontype immutable (experimental)

[TysonAndre]

This reduces memory usage somewhat and makes functions accepting UnionType easier to reason about.

[TysonAndre]

Things to try:

• Can ContextMerge be sped up by checking for identical UnionType instances (And reusing those)
• Can objects (Type->asUnionType()) be reused in withType(), withoutType()
• withType can be sped up for the empty union type
• Might be able to subclass EmptyUnionType to speed analysis up. (Hardcode methods as returning false/true, etc)

[TysonAndre]

Seeing a new bug with this example:

<?php

class Common{
    const TYPE_INT = 'i';
}

class CC extends Common {
    protected static $mappings = [
        'key' => ['name' => 'name', 'type' => self::TYPE_INT],
    ];
}

src/union_type.php:9 PhanUndeclaredConstant Reference to undeclared constant \CC::TYPE_INT