Pre-check wasm before compiling

[ethanfrey]

Let's filter out some stuff before passing it to the backend:

• valid wasm (magic number)
• no floating point ops (wasm-parser can do that)
• check signatures of imports/exports - validate before attempting to instantiate

others??

[ethanfrey]

We can use wasmparser to get some info on the wasm.

This is used in a wasmer PR.

When iterating over all operators, you can force error on any floating point ops

[ethanfrey]

For validating imputs, we can use this library: https://docs.rs/wasm-nm/0.2.1/wasm_nm/fn.symbols.html as a pre-processing step

Get the list of imports and exports. Assert the following exports are all present:

query
init
handle
allocate
deallocate

Ensure that no more than the following imports are required:

i c_read
i c_write
i c_canonical_address
i c_human_address

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