use std as extern crate

[Person-93]

This PR is to prepare for no_std support.

• I've made the main crate no_std and added extern crate std instead.
• I've also added a new top-level mod platform to hold all the upcoming std replacements. As of now, it just has a prelude which contains
  • Things from the alloc crate people generate use from std's prelude
  • the eprintln macro (we will need to find a replacement for this)
• I've added import crate::platform::prelude::* to every file that uses things from the standard prelude.
• The test_utils file could not import the prelude so it had to import directly from the alloc crate.
• in py_format macro: replace use of ToOwned with Clone