Make WithSpanBoxed public (resolves a doc warning)

Author: andyleiserson

naga/src/lib.rs

@@ -134,7 +134,7 @@ pub mod valid;
 use alloc::string::String;
 
 pub use crate::arena::{Arena, Handle, Range, UniqueArena};
-pub use crate::span::{Diagnostic, SourceLocation, Span, SpanContext, WithSpan};
+pub use crate::span::{Diagnostic, SourceLocation, Span, SpanContext, WithSpan, WithSpanBoxed};
 
 // TODO: Eliminate this re-export and migrate uses of `crate::Foo` to `use crate::ir; ir::Foo`.
 pub use ir::*;