Switch back to WebAssembly/binaryen, and first Memory features

1) Switch back to WebAssembly/binaryen

2) First foray into the Land of Memory

added binaryen module validation, thanks to the update to master. Prepared and tested using it to: optimize the module, launch the interpreter (both need CLI options, but work now, and are just commented out). The latter will be useful to integrate with the testing infrastructure
turned off the export attributes for now (even though there’s a good chance we’ll need them eventually :)
refactored trans_rval into a more general assignment trans (similar to miri, and brought in some of its code again) to allow for lvalue knowledge, its type and memory layout (which will for instance be useful to emit more than just i32 binary ops)
this has allowed a prototype implementation of a stack in wasm linear memory (1 static 64k page only for now) by emitting our own function prologue and epilogue to handle the stack pointer. (This at the very least will need to be extracted into its own abstraction, and improved/tested esp. regarding alignments, optimized, etc)
with this, a basic alloca was added to create simple structs (within the current existing limitations around types, i.e. i32s only) emitting store/loads for assignments and access; With impl and refs — and an example using it
added the runtime start fn we talked about in the previous PR, to set up some of our data, and call either the #[start] entry fn with (0, 0), or #[main] directly. This __wasm_start fn will be called automatically on module start

r? @brson @eholk

brson / miri