core: Skip UTF-8 validation in read_value()

The UTF-8 validation is very expensive. It's unclear if we ever need to
do it at read time and if we do, let's do it when we know the value is
actually going to be used. Speeds up read_value() a lot.

core/sqlite3_ondisk.rs

@@ -401,7 +401,8 @@ pub fn read_value(buf: &[u8], serial_type: &SerialType) -> Result<(Value, usize)
             if buf.len() < n {
                 return Err(anyhow!("Invalid String value"));
             }
-            let value = String::from_utf8(buf[0..n].to_vec())?;
+            let bytes = buf[0..n].to_vec();
+            let value = unsafe { String::from_utf8_unchecked(bytes) };
             Ok((Value::Text(value), n))
         }
     }