Fixes How BinaryOperators work on Unions

[markwpearce]

New logic:

• the result of a binary op on unions depends on what types are in the union
• Basically, the "highest priority" type wins
• If two types are "equal priority", but not the same, treat it as Dynamic
• (float or integer) + (Integer) => Float
• Treats "ObjectType" basically like "DynamicType"

[markwpearce]

fixes #1396