Generate proper externs for Closure compiler

[lptr]

Right now we simply collect all symbols used in a Spaghetti API, and tell Closure not to obfuscate those. This is not very efficient (protecting things like "name" or "reference" protects all local variables etc.), and Closure's extern functionality has much more powerful features as well. For example, one can specify the type of a field ('number' or 'string'), which allows closure to optimize better. This should be pretty easy to generate from the Spaghetti definition.