LLVM 13

[csegarragonz]

In this PR, I bump the Faasm ecossytem to use LLVM 13 all around. This means that we use LLVM 13 to cross-compile to WebAssembly, and to build our WASM runtimes (WAVM and WAMR). The latter is usually the trickiest bit, as WASM runtimes implement optimizations (specially when generating AOT binary code), which are tightly coupled with LLVM. In the case of WAMR, its usually a matter of bumping to the latest commit. In the case of WAVM, it involves some fiddling as the library is less maintained.

Below is a list of the requirements for the merge. I keep it for reference, whenever we upgrade the LLVM version again.

To-Do:

• Faasm docs on upgrading the LLVM version: see here.
• Tests passing in PR in faasm/cpp: LLVM 13 cpp#106
• Tests passing in PR in faasm/python: LLVM 13 python#21
• PR in faasm/WAVM: LLVM 13 WAVM#5
• PR in faasm/wasm-micro-runtime: LLVM 13 wasm-micro-runtime#2
• Tests passing in PR in faasm/faabric to remove clang-10: LLVM 13 faabric#285
• WAVM codegen
• WAVM demo/hello
• WAMR codegen
• WAMR demo/hello
• Remove clang-format-10 from all formatting tasks
• Faasm tests running

Before merging:

• Examples building with LLVM 13: LLVM 13 examples#2
• Merge all PRs and update submodules in faasm/faasm

After merging:

• Re-tag examples for latest containers.

Closes #690
Closes #642 (superseeded)

[csegarragonz]

Tests are finally green! 🥲

@Shillaker will request reviews everywhere, you can use this PR's description as guideline.