(WIP) Handle unreachable

[tana]

This PR allows compilation of some WASM codes which previously generated compilation error. In particular, it solves some unreachable-related problems, such as:

• Issue #21 (GreenGood's wasm.zip successfully compiles, aside from lacking imports)
• SpecTest_unreachable (removed [Ignore(StackSizeIncorrectException)] and passes this spec test).

What I did was:

• Stopped ignoring StackSizeException in SpecTest_unreachable.
• Created BlockContext mechanism inside CompilationContext, to remember stack at the beginning of a block and whether the compiling instruction is reachable.
• Skip compilation (including type check) of unreachable instructions.

However, at present, some invalid WASM code involving unreachable (in SpecTest_unreached_invalid) passes compilation, because all unreachable code is now ignored. Therefore, this PR is currently marked Draft.

(Update)

• Skip unreachable instructions following return, br, br_table. (Sorry, commit message of 62d5fd8 is mistake)
  • It allows more WASM codes to compile, but some SpecTests are failing because invalid codes are not detected.

[tana]

I will try to fix the SpecTest_unreached_invalid problem, but because my lack of time, I am not sure when it will complete.

[tana]

I apologize for wasting CPU time of your GitHub Actions CI server. I will refrain from pushing commits until large progress is achieved.

[RyanLamansky]

This project leverages GitHub's hosted actions provider, which is free and effectively unlimited for open source projects like this. Nothing to worry about 🙂

If you want to reduce noise, you could close the PR while you continue to refine your solution and then open a new one.

[tana]

Thank you.

I think PR became a bit dirty (such as wrong commit message). So I decided to close this and make a new one later.