Keep unsafe code forbidden when puffin is disabled (#3603)

This helps document what unsafe is being used for, and prevent other
uses from going unnoticed.

crates/emath/src/lib.rs

@@ -20,7 +20,8 @@
 //!
 
 #![allow(clippy::float_cmp)]
-#![deny(unsafe_code)]
+#![cfg_attr(feature = "puffin", deny(unsafe_code))]
+#![cfg_attr(not(feature = "puffin"), forbid(unsafe_code))]
 
 use std::ops::{Add, Div, Mul, RangeInclusive, Sub};