Support no_std

[theoparis]

There seems to be a std feature that can be disabled with an enabled alloc feature (see https://github.com/wcampbell0x2a/cpio-deku/blob/master/Cargo.toml#L8), but src/lib.rs requires std. I'm not sure what the no-std replacement for read/write would be though - see rust-lang/rust#48331. Most of the alternative read/write trait implementations are all unmaintained (e.g. core2). Also see dtolnay/thiserror#64, thiserror doesn't work with no-std.

[wcampbell0x2a]

This is an unpublished crate (with non release versions of deku), thus I haven't tested it much.

Should be able to get no_std + alloc working. Deku itself supports it.

Deku has an ensure_no_std to make sure this doesn't break. And uses no_std_io to provide Read/Write for no_std.