Restore support for symbol names longer than 65535 bytes

The ClickHouse benchmark contains C++ symbol names longer than the u16
limit, causing valid input to panic in Symbol::new. Restore u32 storage
for the cached name length and retain the checked conversion into that
representation.

Change the boundary test to construct a 65536-byte name successfully and
verify that Symbol::name returns all of its bytes. This removes the
restriction introduced by narrowing the length field.

src/symbol.rs

@@ -233,7 +233,7 @@ pub struct Symbol {
     // in the owner file. Since the symbol map and symbol array are separate,
     // retain a thin pointer rather than a 16-byte slice.
     name_ptr: usize,
-    name_len: u16,
+    name_len: u32,
 
     /// Serializes the parallel updates made while resolving definitions.
     /// The byte also holds the resolution-only `skip_dso` bit; both fit in
@@ -466,7 +466,7 @@ symbol_bits! {
 impl Symbol {
     #[inline]
     pub fn new(name: &'static BStr) -> Symbol {
-        let name_len = u16::try_from(name.len()).expect("symbol name is longer than 65535 bytes");
+        let name_len = u32::try_from(name.len()).expect("symbol name is larger than 4 GiB");
         Symbol {
             name_ptr: name.as_ptr() as usize,
             name_len,
@@ -1909,9 +1909,9 @@ mod tests {
     }
 
     #[test]
-    #[should_panic(expected = "symbol name is longer than 65535 bytes")]
-    fn rejects_too_long_symbol_name() {
+    fn accepts_long_symbol_name() {
         let name = Box::leak(vec![b'x'; u16::MAX as usize + 1].into_boxed_slice());
-        Symbol::new(BStr::new(name));
+        let symbol = Symbol::new(BStr::new(name));
+        assert_eq!(symbol.name(), BStr::new(name));
     }
 }