Add basic exception handling support

[aheejin]

This adds basic support for exception handling instructions, according
to the spec:
https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md

This PR includes support for:

• Binary reading/writing
• Wast reading/writing
• Stack IR
• Validation
• binaryen.js + C API
• Few IR routines: branch-utils, type-updating, etc
• Few passes: just enough to make wasm-opt -O pass
• Tests

This PR does not include support for many optimization passes, fuzzer,
or interpreter. They will be follow-up PRs.

Try-catch construct is modeled in Binaryen IR in a similar manner to
that of if-else: each of try body and catch body will contain a block,
which can be omitted if there is only a single instruction. This block
will not be emitted in wast or binary, as in if-else. As in if-else,
class Try contains two expressions each for try body and catch body,
and catch is not modeled as an instruction. exnref value pushed by
catch is get by pop instruction.

br_on_exn is special: it returns different types of values when taken
and not taken. We make exnref, the type br_on_exn pushes if not
taken, as br_on_exn's type.

[kripken]

Nice work!

[aheejin]

We currently have a Travis CI error which is not reproducible locally, so I added a few debugging commits that comment out some tests or add debug printfs. They will be deleted after debugging.

(Added later: this has been deleted now)

[aheejin]

Talked with @kripken offline and decided do these things:

1. Make exnref, not extracted event type, as br_on_exn's type. This will remove the need for the special validation rule for br_on_exn and also removes the need to use exnref.pop after br_on_exn.
2. Keep event type copy within BrOnExn class. It is necessary if we do 1, because BrOnExn does not have the event type as its type anymore.

Will update the code with this change soon.

[aheejin]

Talked with @kripken offline and decided do these things:

1. Make exnref, not extracted event type, as br_on_exn's type. This will remove the need for the special validation rule for br_on_exn and also removes the need to use exnref.pop after br_on_exn.
2. Keep event type copy within BrOnExn class. It is necessary if we do 1, because BrOnExn does not have the event type as its type anymore.

Will update the code with this change soon.

Updated.

[kripken]

Nice! lgtm, with just some final minor comments/suggestions.

[kripken]

Thanks, lgtm!

[aheejin]

Thank you for the review!