New faster interpreter based on instruction objects and operands.

[aardvark179]

This change implements a new interpreter based on Instruction objects rather than byte codes. Instructions generally have one or two operands which express where they get their data from (literal values, variables, or the stack). Currently all instructions that produce results put those results on the stack, but in the future the operands concept could be extended to where the result is placed and provide further improvements.

This design provides significant performance improvements over the old interpreter (see benchmarks in #2416 ) in return for a slightly increased memory overhead. This overhead is less than might be expected because operand objects can be heavily reused, fewer instructions are usually required for the V2 interpreter, and the byte code itself is often only a portion of the overall foot print of interpreted functions compared to constants, metadata, etc.

While upstreaming this work I've taken the opportunity to remove a lot of duplication between the two interpreters as inconsistencies between these were the source of a few issues while rebasing this work.