Refactor for #![no_std] usage (#2)

* Allow #![no_std] use

Can’t use this crate in a low-level project like an OS kernel without this change.

* Import Vec from alloc crate instead of std

* Use the structures provided by the alloc crate in the macros

Author: kennystrawnmusic

src/lib.rs

@@ -1,4 +1,7 @@
-use std::mem;
+extern crate alloc;
+
+use core::mem;
+use alloc::vec::Vec;
 
 const fn pos<K>(l: &usize, _: &K, _: &Vec<K>) -> usize {
     *l

src/macros.rs

@@ -1,4 +1,6 @@
 use crate::Vectorable;
+use alloc::vec::Vec;
+use alloc::string::String;
 
 #[doc(hidden)]
 #[macro_export]