fix: Wrap each use of filesystem library in #ifndef __wasm__

[phated]

Description

Workaround that fixes #180

As noted in the issue, the <filesystem> library can't be used in code that compiles to Wasm, even if we are targeting WASI. I just quickly wrapped all the usages in #ifndef __wasm__ to verify this workaround solves the problem in Nix. I think the "more correct" way to solve this would be to use mkdir (on unix) and _mkdir (on Windows), but that seemed to receive pushback on the internal PR.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR specifications in /markdown/specs have been updated.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• I've updated any terraform that needs updating (e.g. environment variables) for deployment.
• The branch has been rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.

[adr1anh]

Looks good

[phated]

Since the wasm tests don't run in CI, @dbanks12 recommended I run them locally, so I ran the following commands:

cd build_wasm
for test in ./bin/join_split_example_*tests; do
  wasmtime --dir .. $test;
done

and all succeed 🎉