Make the -sys crate optional

Downstream projects, such as `bitcoin` are using types from this crate
and need to compile the C code even if they only use the types for
checking validity of the data and don't perform cryptographic
operations. Compiling C code is slow and unreliable so there was desire
to avoid this.

This commit introduces pure Rust implementation of basic
non-cryptographic operations (is point on the curve, decompression of
point, secret key constant time comparisons) and feature-gates
cryptographic operations on `secp256k1-sys` which is now optional. To
make use of this, downstream crates will have to deactivate the feature
and possibly feature gate themselves.

The implementation requires a U256 type which was copied from the
`bitcoin` crate. We don't depend on a common crate to avoid needless
compilation when the feature is turned on.

This is a breaking change because users who did cryptographic operations
with `default-features = false` will get compilation errors until they
enable the feature.

Cargo.toml

@@ -18,10 +18,12 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [features]
-default = ["std"]
-std = ["alloc", "secp256k1-sys/std"]
+default = ["sys-std"]
+std = ["alloc"]
+alloc = []
+sys-std = ["std", "sys-alloc", "secp256k1-sys/std"]
 # allow use of Secp256k1::new and related API that requires an allocator
-alloc = ["secp256k1-sys/alloc"]
+sys-alloc = ["secp256k1-sys/alloc"]
 hashes-std = ["std", "hashes/std"]
 rand-std = ["std", "rand", "rand/std", "rand/std_rng"]
 recovery = ["secp256k1-sys/recovery"]
@@ -36,7 +38,7 @@ global-context = ["std"]
 global-context-less-secure = ["global-context"]
 
 [dependencies]
-secp256k1-sys = { version = "0.10.0", default-features = false, path = "./secp256k1-sys" }
+secp256k1-sys = { version = "0.10.0", default-features = false, path = "./secp256k1-sys", optional = true }
 serde = { version = "1.0.103", default-features = false, optional = true }
 
 # You likely only want to enable these if you explicitly do not want to use "std", otherwise enable

contrib/_test.sh

@@ -3,7 +3,7 @@
 set -ex
 
 REPO_DIR=$(git rev-parse --show-toplevel)
-FEATURES="hashes global-context lowmemory rand recovery serde std alloc hashes-std rand-std"
+FEATURES="hashes global-context lowmemory rand recovery serde std alloc hashes-std rand-std secp256k1-sys"
 
 cargo --version
 rustc --version