Allow _ in Wasm exported names
The Anoma validity predicate Wasm signature is:
"_validate_tx": [I64, I64, I64, I64, I64, I64, I64, I64] -> [I64]
So we need to allow exported names containing '_'.
This PR adds a Juvix module that exports a function with this signature
and a test that the resulting Wasm function can be called.
