Add FFI back to default feature set

Gating it behind the `ffi` feature flag would be a breaking change for
3rd party applications which depend on a build-from-source workflow.

Also improve docs while we’re in here.

Author: onethumb

Cargo.toml

@@ -23,6 +23,9 @@ bench = true
 
 [dependencies]
 crc = "3"
+
+# We use digest with default-features = false and features = ["alloc"] to enable heap allocation support in no_std environments.
+# This configuration is safe because the alloc feature in digest does not depend on its default features as of digest v0.10.
 digest = { version = "0.10", optional = true, default-features = false, features = ["alloc"] }
 
 # will be removed once Rust 1.89 is the minimum supported version
@@ -72,13 +75,16 @@ harness = false
 required-features = ["std"]
 
 [features]
-default = ["std", "panic-handler"]
+# default features
+default = ["std", "panic-handler", "ffi"]
 std = ["alloc"] # std implies alloc is available
-cli = ["std"]
 alloc = ["digest"] # marker feature for heap allocation support
-cache = ["alloc", "hashbrown"] # caching requires alloc + hashbrown HashMap
-ffi = ["std"]
 panic-handler = [] # Provides panic handler for no_std library checks (disable in binaries)
+ffi = [] # C/C++ compatible dynamic/static library, planned to become optional in the next MAJOR version (v2) to reduce overhead
+
+# optional features
+cli = ["std"] # command line interface binaries (checksum, arch-check, get-custom-params)
+cache = ["alloc", "hashbrown"] # no_std caching requires alloc + hashbrown HashMap
 
 # the features below are deprecated, aren't in use, and will be removed in the next MAJOR version (v2)
 vpclmulqdq = [] # deprecated, VPCLMULQDQ stabilized in Rust 1.89.0