Implement tableswitch and flexible control flow

[rossberg]

Bikeshedding:

- Currently still named switch, but we can rename it to be super-explicit if preferred.
- For consistency, new jumps are named if_break, case_break, default_break, but we can change that to *_branch or something if we decide to rename the existing break operator as well.

[sunfishcode]

The proposed name in the design repo is br_if; if you don't like it, let's discuss this in the design repo, instead of having you just arbitrarily edit things according to your fancy.

[rossberg]

Relax, I was about to do that. But then saw your latest reply. ;)

[rossberg]

Updated to match WebAssembly/design#427 more closely.

[lukewagner]

For direct correspondence with AstSemantics.md, could you rename to br_if?

[lukewagner]

Generally lgtm with nits addressed. The ast.ml doesn't match what we'd probably want in the binary encoding (a table of indices), but that seems fine since that's already the case for many other nodes (we'll have to define "concrete" AST nodes when we get to binary encoding For Real) and the behavior seems to be equivalent to the table.

[rossberg]

Nits addressed, plus a few other internal naming inconsistencies.

[lukewagner]

Thanks! I think README.md also needs updating of names. I'd wait to here what @sunfishcode thinks before merging though :)

[rossberg]

Closing; superseded by #163.