flow2: snap

Change-Id: I8edbe35a9fbb7f807295468a6ee70ff87e757f12
[flow] big chunk of work towards a better Flow Control Framework.

It's not complete of course, the match-keyword is missing on all ends
(syntax, AST, sema, codegen, VM), but the rest is ~80% complete (bugs ignored).

Change-Id: I9e9a07dca310db1ad1bf0b212f52cd4fedf593d5