Always set #![no_std] to fix redundant import warning

[taiki-e]

error: the item `Vec` is imported redundantly
   --> src/lib.rs:119:5
    |
119 | use alloc::vec::Vec;
    |     ^^^^^^^^^^^^^^^
   --> /rustc/5119208fd78a77547c705d1695428c88d6791263/library/std/src/prelude/mod.rs:115:13
    |
    = note: the item `Vec` is already defined here
    |
    = note: `-D unused-imports` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unused_imports)]`