Byteyarn: Add no_std support

[an-owl]

Most of this PR is just changing std to core/alloc.

A couple off modules imported std::alloc which creates a problem when using anything from liballoc, so I used ::alloc::.. in a number of places, IMO a bit smelly but the easiest and least invasive option.

Usually no_std compatible crates include a "std" default feature flag to enable libstd but nothing in byteyarn actually uses libstd so I didn't bother. This may create a breaking change in the future if libstd is ever required so I can add a feature flag if you want.

pub mod m {
    pub extern std;
}

is in lib.rs, for "Macro stuff" which I've changed to pub extern crate core as std; for fear of breaking things. From what I can see this is required by the yarn! macro. It may be better to remove this entirely and change yarn! to use ::core::fmt::format_args! instead.