Stateless Feature (#265)

* add stateless feature and tests

* update docs and new function

Cargo.lock

@@ -993,6 +993,15 @@ dependencies = [
  "subtle",
 ]
 
+[[package]]
+name = "document-features"
+version = "0.2.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb6969eaabd2421f8a2775cfd2471a2b634372b4a25d41e3bd647b79912850a0"
+dependencies = [
+ "litrs",
+]
+
 [[package]]
 name = "ecdsa"
 version = "0.16.7"
@@ -1611,6 +1620,12 @@ version = "0.4.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "da2479e8c062e40bf0066ffa0bc823de0a9368974af99c9f6df941d2c231e03f"
 
+[[package]]
+name = "litrs"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b4ce301924b7887e9d637144fdade93f9dfff9b60981d4ac161db09720d39aa5"
+
 [[package]]
 name = "lock_api"
 version = "0.4.9"
@@ -2281,6 +2296,7 @@ dependencies = [
  "cfg-if",
  "color-eyre",
  "criterion 0.4.0",
+ "document-features",
  "lazy_static 1.4.0",
  "num-bigint",
  "num-traits",