Fix emscripten demo and libwabt.js (#407)
This removes the more complicated emscripten wrapper we used before, in
favor of something that is much simpler.
* Remove the memory init file
* Remove the onInputKey handling in the demo so it doesn't auto-indent
(it was broken now that we have the flat syntax)
* Simplify emscripten-helpers to provide very simple C-function wrappers
around the wast2wasm API:
* wabt_parse_ast
* wabt_resolve_names_script
* wabt_validate_script
* wabt_write_binary_module
The tricky part is that some functions return multiple values, so
those are returned as structs as well:
* WabtParseAstResult
* WabtWriteBinaryModuleResult
