Make this crate no_std

As almost nothing actually depends on std, there is no need to keep a
dependency on it.  This allows users such as the jid crate to actually
be no_std as well.

The only exception is std::error::Error, which is now exposed as
core::error::Error in nightly but not yet stabilized, see
rust-lang/rust#103765

Cargo.toml

@@ -11,3 +11,7 @@ readme = "README.md"
 unicode-bidi = "0.3"
 unicode-normalization = "0.1"
 unicode-properties = "0.1.1"
+
+[features]
+default = ["std"]
+std = []

src/lib.rs

@@ -1,13 +1,18 @@
 //! An implementation of the "stringprep" algorithm defined in [RFC 3454][].
 //!
 //! [RFC 3454]: https://tools.ietf.org/html/rfc3454
+#![no_std]
 #![warn(missing_docs)]
+extern crate alloc;
+#[cfg(feature = "std")]
+extern crate std;
 extern crate unicode_bidi;
 extern crate unicode_normalization;
 extern crate unicode_properties;
 
-use std::borrow::Cow;
-use std::fmt;
+use alloc::borrow::Cow;
+use alloc::string::String;
+use core::fmt;
 use unicode_normalization::UnicodeNormalization;
 use unicode_properties::{GeneralCategoryGroup, UnicodeGeneralCategory};
 
@@ -44,6 +49,11 @@ impl fmt::Display for Error {
     }
 }
 
+// TODO: We can remove this feature check and use core::error::Error directly once it gets
+// stabilized, see https://github.com/rust-lang/rust/issues/103765
+//
+// For now we could use it in nightly with #![feature(error_in_core)] but hopefully not for long.
+#[cfg(feature = "std")]
 impl std::error::Error for Error {}
 
 /// Prepares a string with the SASLprep profile of the stringprep algorithm.

src/tables.rs

@@ -1,6 +1,6 @@
 //! Character Tables
-use std::cmp::Ordering;
-use std::str::Chars;
+use core::cmp::Ordering;
+use core::str::Chars;
 use unicode_bidi::{bidi_class, BidiClass};
 use unicode_properties::{GeneralCategoryGroup, UnicodeGeneralCategory};