Allows pushing Lua code

[tomaka]

This PR allows pushing Lua code.

For example:

lua.set("foo", hlua::functions_read::LuaCode("return 5"));

You can then do:

let r = lua.execute_code::<i32>("foo()").unwrap();
assert_eq!(r, 5);

Similar to LuaCode, also adds LuaCodeFromReader.

This change required modifying the Push trait to return a Result instead, in order to handle syntax errors. More code cleanups will need to be made later. Since the code was quite dirty already, I don't think it's a problem to add even more panicking corner cases.

[tomaka]

While it is maybe controversial to modify Push to return a Result, let's look at the alternatives:

• Not add this feature. This would be unfortunate as there's currently no way to put a Lua function to a variable.
• Use two steps: put the function on the stack with a custom method, then write it, with two different methods. However this causes tons of problems, like how to handle the fact that the user could pop something under the stack before poping the function at the top.
• Panic if there's a syntax error. Meh.

I'm going to change type Err = (); into type Err = Void; where Void is a non-instantiable type.

[tomaka]

The idea now is that set can still be used for anything except LuaCode and LuaCodeFromReader, while the new checked_set functions can be used with everything and returns a Result.