WASM-related dependencies and feature cleanup

[igamigo]

After #367 gets merged, we need to address the following:

• Remove the default features:

  default = ["std", "dep:hex", "dep:thiserror", "dep:prost"]

  ~None of these should be included by default~ Only std should remain here. For this, we need to get rid of the Miden node's duplicate code (see context). We should also be good to remove the related dependencies after that.

• After this, we should also be good to add something like this to the library root:

  #![no_std]
  
  #[macro_use]
  extern crate alloc;
  
  #[cfg(feature = "std")]
  extern crate std;

[bobbinth]

default = ["std", "dep:hex", "dep:thiserror", "dep:prost"]

None of these should be included by default. For this, we need to get rid of the Miden node's duplicate code (see context).

One small correction: we should include only std by default. This way, --no-default-features flag would enable no_std mode.