WIP/POC AVM2 verifier

[Lord-McSweeney]

This is based off @adrian17's original work on his peephole optimizer branch (see #7920 and a newer branch)

This results in noticeable performance increases on #10560 (6-7% decrease in startup time and even more, clearly noticeable improvements in gameplay) and in #12303.

This supersedes the main part of #14496, the opcode reading optimizations.

TODO:

• Something in try..catch blocks is broken (I keep losing working reproductions, but FP is throwing verify errors where Ruffle is not)
  • Also only verify exception target if there are any potentially error-throwing opcodes present in the from..to
  • All error-throwing opcodes in the from..to should act as a potential jump to the exception target (just like iftrue, iffalse, etc)- example: verify.zip
• More brokenness
• Add many tests
• Store parsed_code on the method itself rather than in AbcMethodBody
• Add a custom Op type in AVM2 that stores already-resolved constant pool entries
• Storing an entire verified AbcMethodBody seems unnecessary, and is mostly leftover from Adrian's original work

This allows for further optimizations that actually modify the bytecode, such as #7920.

[evilpie]

Nice. I imagine shrinking the Opcode size would improve this even more.

[Lord-McSweeney]

Closing in favor of #14631.