Update exception handling support to current proposal

[takikawa]

This PR updates the support of exception handling to the latest proposal (that is compatible with future 2-phase exception handling) described in WebAssembly/exception-handling#137 and WebAssembly/exception-handling#143.

• Adds back tagged catch $e, catch_all, and rethrow N from a previous version of wabt, but with updates to match the current spec (e.g., catch_all shares an opcode with else, rethrow's depth indexes only catch blocks, etc).
• Adds unwind and delegate instructions.
• Removes exnref and br_on_exn.
• Updates relevant tests.

There are some details that could still change (e.g., maybe how delegate's depth is validated), but I'd be happy to submit further PRs if the spec details change.

[binji]

lgtm, thanks for doing this! Sorry for the long delay on review

[dschuff]

I just noticed (after the discussion in #1604, thanks for pointing it out) that this PR doesn't actually solve the problem that made me notice the opcode problem, (because I was only using the disassembler). Namely wasm-objdump prints opcode 0x5 as else rather than catch-all even in the context of try. Presumably it's just going directly from opcode.def, (which doesn't have catch_all in this PR). But maybe let's not fix this until we decide whether we want to keep the opcodes as they are or change them.

[aheejin]

Thanks @takikawa! By the way can I merge this? If the opcode is the issue we can fix it later.

[takikawa]

@aheejin Sure, no issues on my end with having it merged. AFAIK I've addressed all the comments in review. Thanks!

I'd also be happy to submit a PR afterwards if the opcode changes.