Add runtime_detection feature

Runtime CPU detection currently requires `std`, so by unconditionally enabling it, we further block the possibility of adding `no_std` support. To ensure the default performance for users is unaffected, `runtime_detection` is added to the set of default features.

According to the [Cargo Book](https://doc.rust-lang.org/cargo/reference/semver.html#cargo-change-dep-feature), this can be considered a minor change.

Author: bushrat011899

Cargo.toml

@@ -21,12 +21,11 @@ exclude = [".*"]
 [dependencies]
 libz-sys = { version = "1.1.20", optional = true, default-features = false }
 libz-ng-sys = { version = "1.1.16", optional = true }
-# this matches the default features, but we don't want to depend on the default features staying the same
-zlib-rs = { version = "0.6.0", optional = true, default-features = false, features = ["std", "rust-allocator"] }
+zlib-rs = { version = "0.6.0", optional = true, default-features = false, features = ["rust-allocator"] }
 ## This implementation uses only safe Rust code and doesn't require a C compiler.
 ## It provides good performance for most use cases while being completely portable.
 miniz_oxide = { version = "0.9.0", optional = true, features = ["simd"] }
-crc32fast = { version = "1.2.0", optional = true }
+crc32fast = { version = "1.2.0", default-features = false, optional = true }
 document-features = { version = "0.2", optional = true }
 
 [dev-dependencies]
@@ -37,7 +36,10 @@ quickcheck = { version = "1.0", default-features = false }
 [features]
 ## The default backend using pure Rust implementation via miniz_oxide.
 ## This provides a safe, portable compression implementation without requiring a C compiler.
-default = ["rust_backend"]
+default = ["rust_backend", "runtime_detection"]
+
+## Allows runtime feature detection on supported CPU architectures.
+runtime_detection = ["zlib-rs?/std", "crc32fast?/std"]
 
 #! ### User-Facing Backend Features
 #! Choose one of these features to select the compression backend.