Support rust-std in WebAssembly runtime

[sorpaas]

The target we compile wasm runtime to, wasm32-unknown-unknown, actually has builtin std support. Adding support for std would help to solve many of our current user complaints.

Some details of the std support on the wasm target:

• Filesystem is implemented as shim that cannot be constructed.
• Single thread is assumed, which is used to implement constructs like Mutex and RwLock.
• You can find more about the std implementation at this folder.
• I think the only issue we need to deal with is the allocator. Currently the std library assumes dlmalloc, but it should be possible to switch it to our external allocator.

[sorpaas]

After some discussions with @gavofyork I think we'll not be adding std support in runtime. Closing the issue.

[h4x3rotab]

Why? I think any background here about the cancellation would be helpful for the developers.

[sorpaas]

@h4x3rotab I think the general consensus yesterday was that:

• If developers try to do stuff which are not allowed in runtime, std opens up some caveats of panics, potential DoS attacks, among others. This is something we don't want to see.
• While it was not the intention, we wrote many code with the assumption that std is native, no_std is wasm.
• It makes things clear that runtime must be deterministic. It may not be a "perfect model", and I do think there're other models that may work better (for example, post-build check of unexpected imports), but still, this is a model that works.
• It's considered by many that wasm32-unknown-unknown's std implementation is just unstable and not for production, while wasm itself is.