Differentiate no-std from alloc within tests/no-std

Fixes `no-std` builds for packages which intended to be `no-std` (without
`alloc`).

Updates a variety of MSRVs to 1.73 due to `flexible-transcript` no longer using
`std-shims` to achieve 1.66 (as `std-shims` requires `alloc`). A future
improvement would be for `std-shims` to have an `alloc` feature and only
provide MSRV shims without it.

crypto/transcript/src/lib.rs

@@ -2,9 +2,6 @@
 #![doc = include_str!("../README.md")]
 #![no_std]
 
-#[allow(unused_imports)]
-use std_shims::prelude::*;
-
 use zeroize::Zeroize;
 
 use digest::{