binaryen.js improvements

[kripken]

Some stuff I noticed when writing some algorithms from scratch using binaryen.js for fun:

• The default type of a block should be none, not unreachable. This is technically a breaking change, but the previous way is just not good (and was not tested).
• Allow a more spec-like notation, e.g. br as an alias for break, get_local as an alias for getlocal, etc., and add a br_if. This keeps backwards compatibility, both old and new ways work.

[kripken]

cc @dcodeIO

[dcodeIO]

LGTM (never relied on binaryen.undefined as the default in blocks)