Make cargo-nono alloc-aware

[hobofan]

Consists of multiple things:

• Should only be triggered with an --allow-alloc flag; By default the alloc should not be assumed to be present on target systems.
• Detecting replacing std with alloc similar to this: https://github.com/CraneStation/cranelift/blob/4e096e29cecc762e345cfc62c34e3113ba5777d7/lib/wasm/src/lib.rs#L30-L38 . If that is detected, filter out all use std:: offenses that have an equivalent valid use alloc:: statement.
• Write a guide/documentation on that pattern, and point users to that when a use related the alloc crate (e.g. use std::vec::Vec is detected)