Allow this crate to work with `no_std`

[bcattle]

Attempting to install this crate with cargo produces the error

$ cargo build
    Updating crates.io index
   Compiling counted-array v0.1.2
error[E0463]: can't find crate for `std`
  |
  = note: the `thumbv6m-none-eabi` target may not support the standard library
  = note: `std` is required by `counted_array` because it does not declare `#![no_std]`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: could not compile `counted-array`

To learn more, run the command again with --verbose.

However the crate does not need no_std to work.

This crate is supremely useful in a no_std environment, where you have e.g. long lists of configuration bytes stored in const arrays.