It's much better to tell the user "hey, the magic numbers don't check
out" than "oh there was a problem with your input" :P
Also refactors some stuff to make it possible to efficiently use the
parser error enum without it getting in the way.
This can currently parse a really simple module.
Note that it cannot parse the DataCount section, and it's still missing
almost all of the instructions.
This commit also adds a 'wasm' test utility that tries to parse a given
webassembly binary file.
It currently does nothing but exit when the parse fails, but it's a
start :^)
