Java Skip runtime support

[bennostein]

	* JNA FFI w/ dynamic proxies? compile-time codegen? codegen seems preferable from perf perspective
	* JSON-serializable skip-heap objects through our constructors for skip-heap data etc.
	* best-effort checking that mappers themselves don't have mutable state: more reasonable than proper guarantees!
		* fields must either (a) be final & primitive or (b) <: our superclass
		* possible hurdle: build systems that link javac, which makes it potentially tricky to use annotation processors
		* warn/check against egregious misuse of mutable globals; don't fully walk callees