Deserializing now borrows symbols appropriately

I noticed that SymbolMap::new() returned an Owned map, which prevented
the cheaper option of borrowing when possible. I updated both from_slice
and from_read, but the from_read path can't actually call the borrow
push function due to not being able to borrow.

I also noticed that tuple serialization wasn't tested, although enum
tuples were.

pot/src/de.rs

@@ -39,7 +39,11 @@ impl<'s, 'de> Deserializer<'s, 'de, SliceReader<'de>> {
     /// Returns a new deserializer for `input`.
     #[inline]
     pub(crate) fn from_slice(input: &'de [u8], maximum_bytes_allocatable: usize) -> Result<Self> {
-        Self::from_slice_with_symbols(input, SymbolMap::new(), maximum_bytes_allocatable)
+        Self::from_slice_with_symbols(
+            input,
+            SymbolMap::Borrowed(SymbolList::new()),
+            maximum_bytes_allocatable,
+        )
     }
 
     fn from_slice_with_symbols(
@@ -64,7 +68,7 @@ impl<'s, 'de, R: ReadBytesExt> Deserializer<'s, 'de, IoReader<R>> {
     pub(crate) fn from_read(input: R, maximum_bytes_allocatable: usize) -> Result<Self> {
         Self::new(
             IoReader::new(input),
-            SymbolMap::new(),
+            SymbolMap::Borrowed(SymbolList::new()),
             maximum_bytes_allocatable,
         )
     }

pot/src/lib.rs

@@ -338,6 +338,11 @@ mod tests {
         test_serialization(&0.1_f32, Some(5));
     }
 
+    #[test]
+    fn tuples() {
+        test_serialization(&(1, true, 3), None);
+    }
+
     #[test]
     fn enums() {
         test_serialization(&EnumVariants::Unit, None);