Make flatc generate Rust files not requiring std

[fjarri]

flatc generates Rust accessors for flatbuffers structures using std imports, which prevents it from being used in Rust's no_std environment. This PR replaces std imports with the equivalent core ones, which do work in no_std.

For reviewers: this is mostly a search-and-replace PR, I did not get into the architecture of the compiler, so I might have missed some subtle appearances of std (e.g. if a string is assembled out of several variables or something) - please check.

[CasperN]

Can you

1. add a test binary in tests/rust_usage_test/ (or maybe make another test crate tests/rust_no_std_tests/) that;
2. uses ![no_std],
3. imports the generated code, and
4. call those tests in tests/RustTest.sh?

This way, CI will confirm that we can at least build with no_std.

[fjarri]

So, it turned out RustTest.sh already had a cargo test --features no_std, but it didn't have any effect, since the only thing that the no_std feature changed was the allocator in integration_tests.rs. So I added an explicit !#[no_std], and gated std usage on the lack of no_std feature. Had to also gate a few tests that used the filesystem, but the majority is still working.

It is still a mystery how that test worked before #7268, since for me it immediately fails with a compilation error.

[CasperN]

It is still a mystery how that test worked before #7268, since for me it immediately fails with a compilation error.

Yeah, that's a little sus. Also I found the out_dir test was broken (sigh), I forgot it was disabled on CI

[fjarri]

Great, would you be able to publish a new release of flatc in https://github.com/google/flatbuffers/releases/? (this will also close #7081)

I think it should be a patch version bump; it will break the users somehow still on rust_edition=2015, but there should be a negligible quantity of those. Edit: actually, scratch that, since flatbuffers crate uses 2018 edition, I don't think it is possible to use it in a crate with 2015.