Update changelog for hslua v0.8.0
## Changelog
+### 0.8.0
+
+- Use newtype definitions instead of type aliases for LuaNumber and LuaInteger.
+ This makes it easier to ensure the correct numeric instances in situations
+ where Lua might have been compiled with 32-bit numbers.
+- Instances of `FromLuaStack` and `ToLuaStack` for `Int` are removed. The
+ correctness of these instances cannot be guaranteed if Lua was compiled with a
+ non-standard integer type.
+
### 0.7.1
- The flag `lua_32bits` was added to allow users to compile Lua for 32-bit
