add erc721 feature and module

contracts/Cargo.toml

@@ -21,6 +21,7 @@ wavm-shims = { path = "../lib/wavm-shims" }
 [features]
 default = []
 erc20 = []
+erc721 = []
 
 [lib]
 # The Stylus team sets new crates with the following types:

contracts/src/erc721/mod.rs

@@ -0,0 +1 @@
+

contracts/src/lib.rs

@@ -9,6 +9,8 @@ static ALLOC: mini_alloc::MiniAlloc = mini_alloc::MiniAlloc::INIT;
 
 #[cfg(any(test, erc20))]
 pub mod erc20;
+#[cfg(any(test, erc721))]
+pub mod erc721;
 #[cfg(test)]
 mod test_utils;