Rename `schnorrsig` module to `schnorr`

Schnorr is commenly known as a signature algorithm, we don't need
to restate that in the name of the module.

src/lib.rs

@@ -148,7 +148,7 @@ mod key;
 pub mod constants;
 pub mod ecdh;
 pub mod ecdsa;
-pub mod schnorrsig;
+pub mod schnorr;
 #[cfg(feature = "serde")]
 mod serde_util;