Zero the memory of the Wasm VM

[tomaka]

Fix #946

Unfortunately the full node syncing speed is now abysmal. Need to figure out what happens.

[tomaka]

Apparently Substrate restores what's below __heap_pages instead of zero-ing what's above.

[tomaka]

For context I haven't merged this, because the last time I tried these changes considerably slow down the blocks verification speed of the full node (something like 50 times slower if I remember correctly), which means that I'm probably missing something that Substrate does or doesn't do.