sasl: Make this crate no_std

We mostly had to import from alloc some structs that are part of the std
prelude, such as Vec and String.

sasl/src/server/mechanisms/anonymous.rs

@@ -1,5 +1,7 @@
 use crate::common::Identity;
 use crate::server::{Mechanism, MechanismError, Response};
+use alloc::format;
+use alloc::vec::Vec;
 
 use getrandom::getrandom;
 

sasl/src/server/mechanisms/plain.rs

@@ -1,6 +1,8 @@
 use crate::common::Identity;
 use crate::secret;
 use crate::server::{Mechanism, MechanismError, Response, Validator};
+use alloc::string::String;
+use alloc::vec::Vec;
 
 pub struct Plain<V: Validator<secret::Plain>> {
     validator: V,

sasl/src/server/mechanisms/scram.rs

@@ -1,4 +1,8 @@
-use std::marker::PhantomData;
+use alloc::borrow::ToOwned;
+use alloc::format;
+use alloc::string::{String, ToString};
+use alloc::vec::Vec;
+use core::marker::PhantomData;
 
 use base64::{engine::general_purpose::STANDARD as Base64, Engine};
 

sasl/src/server/mod.rs

@@ -1,6 +1,8 @@
 use crate::common::Identity;
 use crate::secret::Secret;
-use std::fmt;
+use alloc::vec::Vec;
+use core::error::Error;
+use core::fmt;
 
 #[cfg(feature = "scram")]
 use crate::common::scram::DeriveError;
@@ -171,7 +173,6 @@ impl Error for ProviderError {}
 
 impl Error for ValidatorError {}
 
-use std::error::Error;
 impl Error for MechanismError {
     fn source(&self) -> Option<&(dyn Error + 'static)> {
         match self {