Support no_std

[ivila]

Feature request

make safetensors crate support no_std feature.

Motivation

Safetensors is used by some other crates, and some of them support no_std feature, for example, the dfdx, however, currently safetensors do not support no_std, so people cannot use dfdx to export model at no_std environment.

Your contribution

Currently I make a patch in my own git repository, adding a no_std feature and:

1. when enable no_std, swtich from HashMap to hashbrown::HashMap
2. change some types or macros from std to alloc:
   1. String => alloc::string::String
   2. format! => alloc::format!
   3. Vec => alloc::vec::Vec
   4. vec! => alloc::vec!
   5. std::borrow::Cow => alloc::borrow::Cow
3. change some traits or types from std to core:
   1. std::ops => core::ops
   2. std::fmt::Display => core::fmt::Display
   3. std::error::Error => core::error::Error
   4. std::str => core::str
4. switch serde and serde_json to no_std
5. when enable, remove some dependencies of std:
   1. remove the IoError in std::io::Error
   2. remove methods that loads from or writes to file (serialize_to_file)

I would like to create a PR if you allow.