Exclude lexical code when built without float_roundtrip

Author: dtolnay

src/de.rs

@@ -1,6 +1,7 @@
 //! Deserialize JSON data to a Rust data structure.
 
 use crate::error::{Error, ErrorCode, Result};
+#[cfg(feature = "float_roundtrip")]
 use crate::lexical;
 use crate::lib::str::FromStr;
 use crate::lib::*;

src/lib.rs

@@ -455,6 +455,7 @@ mod features_check;
 mod io;
 #[cfg(feature = "std")]
 mod iter;
+#[cfg(feature = "float_roundtrip")]
 mod lexical;
 mod number;
 mod read;