Fix no-std compatibility

[Systemcluster]

no-std support is currently non-functional. This PR does the following:

• Replace the unconditional std imports with core and alloc counterparts
• Remove most of the if_std! gates (Box, String and Vec are available in alloc)
• Allow exporting classes without std
• Add a check --no-default-features call to the check workflow

There are no behavioral changes.

Note: I assume wasm-bindgen with std disabled was initially supposed to be alloc-free, but considering it hasn't been functional without std for a while at all, I don't think it's necessary to keep that limitation. If necessary, a separate alloc feature instead might make sense.

externref, intern and take_last_exception/__wbindgen_exn_store remain std-gated due to use of std::thread_local!.

[daxpedda]

Apologies for the delay!
Thank you for doing this!

[Systemcluster]

Thanks for merging!