Get rid of borsh::maybestd

[kpp]

Description

This is a part of upgrade to a new version of borsh.

Remove line pub use borsh::maybestd::{borrow, boxed, collections, format, io, string, vec}; and use alloc crate whenever possible. Instead of borsh::maybestd::hashmap use hashbrown. Instead of borsh::maybestd::io use no_std_io crate.

With a new version of borsh it will be possible to use borsh::io module instead of no_std_io crate.

Testing

make test

[eyusufatik]

I think using std::Vec etc. might break zk vm compatability. Are you sure about your changes?

[kpp]

Alloc::vec::Vec is designed for no_std. Borsh::maybestd reexports Vec from alloc, I did it transparently to reduce code dependency from borsh. In version 1 borsh doesnt have that maybestd module.
And we have ‘make check-nostd’ which performs checks in a nostd env for risc0. Those particular std vecs are okay because they are used along with std Result. It would break on Result first way before vecs.

[kpp]

Fixed by #851