Merge rust-bitcoin#3140: primitives: Fix alloc feature

3b5bcd0 primitives: Fix alloc feature (Tobin C. Harding)

Pull request description:

  We have an `alloc` feature but we are unconditionally using `extern crate alloc`, this is broken - clearly we need to add a `no-std` build for `primitives` in CI.

  Feature gate the `alloc` crate.

  While we are at it just pull types in from `alloc` in the `prelude` - I have no idea why we do not do this in `bitcoin`?

ACKs for top commit:
  Kixunil:
    ACK 3b5bcd0
  apoelstra:
    ACK 3b5bcd0 successfully ran local tests

Tree-SHA512: f042dfb368dc0bde16237764a0dd09def7eeeb59291a5dbb1b5bd5f1dcd49b3adf66c12508c3be4f0ff48b52cd31cbb20edea68d9d55760e99c506ac5b114781

Author: apoelstra

primitives/src/lib.rs

@@ -18,6 +18,7 @@
 #![allow(clippy::manual_range_contains)] // More readable than clippy's format.
 #![allow(clippy::needless_borrows_for_generic_args)] // https://github.com/rust-lang/rust-clippy/issues/12454
 
+#[cfg(feature = "alloc")]
 extern crate alloc;
 
 #[cfg(feature = "std")]
@@ -44,9 +45,6 @@ pub use self::sequence::Sequence;
 #[rustfmt::skip]
 #[allow(unused_imports)]
 mod prelude {
-    #[cfg(all(not(feature = "std"), not(test)))]
+    #[cfg(feature = "alloc")]
     pub use alloc::string::ToString;
-
-    #[cfg(any(feature = "std", test))]
-    pub use std::string::ToString;
 }