runtime: Partially implement box-lambda and unbox-lambda experimental opcodes

These opcodes are not yet fully specified, and *will* change before they become shippable.
Do not write production code against experimental opcodes.

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Implement partial interpreter support for new dex instructions box/unbox-lambda.
* box-lambda will take a closure and convert it into an Object
* unbox-lambda will take an Object and convert it to a closure

(Currently does not implement object identity or variable capture).

All new opcodes are disabled by default, use runtime option -Xexperimental-lambdas to enable them.

Change-Id: I3c15ccf8a26ccecd1d35808a8c1b4149220f6019