"life before invoke" in wasm code

[Stebalien]

Currently, code can run before invoke, but it'll run out of gas immediately and trap (leading to a fatal error).

We need to either:

1. Properly account for this kind of code.
2. Forbid it.

Ideally, we'd just forbid it. It looks like we can run wasm-ctor-eval (from binaryen)?

[raulk]

@Stebalien Is this talking specifically about Wasm module start functions? What other bytecode can exist before invoke in our case?

[Stebalien]

1. The start function.
2. Table initialization functions. See the exprs that appear in https://webassembly.github.io/spec/core/syntax/modules.html#syntax-elem.

[Stebalien]

I'm centralizing all wasm validation in #1038.