Improve manifest whitespace

Mirror the whitespacing in `rust-bitcoin` by doing:

- Only use single line of whitespace between sections
- Separate optional dependencies from non-optional ones

Cargo.toml

@@ -37,11 +37,11 @@ global-context-less-secure = []
 
 [dependencies]
 secp256k1-sys = { version = "0.4.2", default-features = false, path = "./secp256k1-sys" }
+
 bitcoin_hashes = { version = "0.10", optional = true }
 rand = { version = "0.6", default-features = false, optional = true }
 serde = { version = "1.0", default-features = false, optional = true }
 
-
 [dev-dependencies]
 rand = "0.6"
 rand_core = "0.4"