Feature request: Allow an ext code var to be initialized using an ast.Node

I have a use-case where a piece of jsonnet code that is costly to eval returns an object and I want to use this object for subsequent invocations of other files in separate VMs as an external code var. This object is big and not everything that is subsequently eval-ed needs to use it.

Each eval runs in an isolated VM partly because of concurrent processing of components and partly because every invocation has a different set of vars which makes the import cache useless anyway. That is, instead of re-using VMs in complex ways taking concurrency into account etc., my code just creates a fresh VM every time.

What would be really helpful is if I could turn the initial object to an `ast.Node` using `SnippetToAST` and initialize it as `vm.ExtCodeNode(name, node)` or something.

Passing in the serialized version of the object negates the gains I've made in the preprocessing in that the extVar is eagerly turned into a node every time even if the component code doesn't happen to require the value of the var. And being a giant object, it happens to be costly to parse.

I hacked together some crappy code to implement `ExtCodeNode` in my local vendor tree and verified that what I'm describing is indeed the bottleneck that makes everything slow.

Other ideas also welcome.