Use core as much as possible, add no_std attr

[lxrst]

This crate is much more minimal compared to most alternatives. Some std imports can be core instead. Also, while this crate is not no_std-compatible, using the attribute disables the libstd prelude, forcing manual imports. I find this to be a good reminder of reliance on libstd.

[mrlazy1708]

Thanks for this! Though I'm bit confused what are the possible use cases for this? Do I expect some user to have this in Cargo.toml ?

std=some-custom-implementation
tqdm="0.6"

[lxrst]

what are the possible use cases for this?

Potentially smaller binaries; libstd is quite a bit heavier than libcore.

For example, making an assembler smaller than nasm:

Do I expect some user to have this in Cargo.toml?

No, it has no effect on the end user whatsoever.

[mrlazy1708]

I tried your branch, but it seems that the binary has almost the same size as the binary produced by current HEAD (tried both w/ and w/o the release tag). I figured that crossterm has std included so removing std on my side has little effect.