Add read/write capabilities to alloc feature when std not enabled

[leighmcculloch]

No description provided.

[renfengshi]

We need decode stellar transactions in Keystone device, which only supports no_std.
When can I use the feature of no_std?

[renfengshi]

We are developing right now, and blocked by the no_std. May I get some help from you?

[overcat]

I want to provide its read and write capabilities in no_std based on https://crates.io/crates/embedded-io

It provides the following interfaces, which can facilitate us to maximize the reuse of existing code, and it has a large number of downloads.

use embedded_io::{BufRead, ErrorType, Read, Write};