Add std feature, allow crate to work in #![no_std] mode

Disabling the `std` feature (by setting `default-features = false` for
this crate in the Cargo.toml of something requiring it) will enable the
crate to work without the standard library, assuming the presence of
the `alloc` crate and a working global allocator.

We've explicitly imported the individual `alloc` items required by this
crate, rather than using `#![feature(alloc_prelude)]` as `alloc_prelude`
requires Rust nightly, and doing it that way would be a breaking change
to the MSRV.

Tested with Cargo/rustc 1.53.0

Cargo.toml

@@ -12,3 +12,7 @@ categories = [
     "command-line-interface",
     "parser-implementations"
 ]
+
+[features]
+std = []
+default = ["std"]

README.md

@@ -11,6 +11,10 @@ specially, which I believe is more compliant.
 The algorithms in this crate are oblivious to UTF-8 high bytes, so they iterate
 over the bytes directly as a micro-optimization.
 
+Disabling the `std` feature (which is enabled by default) will allow the crate
+to work in `no_std` environments, where a the `alloc` crate, and a global
+allocator, are available.
+
 # LICENSE
 
 The source code in this repository is Licensed under either of

src/lib.rs

@@ -14,13 +14,25 @@
 //!
 //! The algorithms in this crate are oblivious to UTF-8 high bytes, so they iterate over the bytes
 //! directly as a micro-optimization.
+//!
+//! Disabling the `std` feature (which is enabled by default) will allow the crate to work in
+//! `no_std` environments, where a the `alloc` crate, and a global allocator, are available.
 
-use std::borrow::Cow;
+#![cfg_attr(not(feature = "std"), no_std)]
+
+extern crate alloc;
+use alloc::vec::Vec;
+use alloc::borrow::Cow;
+use alloc::string::String;
+#[cfg(test)]
+use alloc::vec;
+#[cfg(test)]
+use alloc::borrow::ToOwned;
 
 /// An iterator that takes an input string and splits it into the words using the same syntax as
 /// the POSIX shell.
 pub struct Shlex<'a> {
-    in_iter: std::str::Bytes<'a>,
+    in_iter: core::str::Bytes<'a>,
     /// The number of newlines read so far, plus one.
     pub line_no: usize,
     /// An input string is erroneous if it ends while inside a quotation or right after an