Add no_std compatibility

[AbdelStark]

Description

This PR enables no_std compatibility for starknet-rs.
It uses no-std-compat crate to achieve this goal.

Usage

If you want to use a crate from starknet-rs with no-std support you can do like this:

starknet-crypto = { git = "...", rev = "...", default-features = false }

std = [
  "starknet-crypto/std",
]

[AbdelStark]

Actually it seems we won't be able to use no-std-compat.

We will likely have to handle the compatibility manually, like what Substrate std is doing:
https://github.com/paritytech/substrate/blob/polkadot-v0.9.37/primitives/std/src/lib.rs