Add metamethods for bitwise operators and remove `__gc`

GPG added metamethods for bitwise operators which I didn't do when I added them. The methods are called
```c
"__band", "__bor", "__bshl", "bshr"
```

They also removed the `__gc` metamethod.

For inspiration only, the implementation here should not necessarily be taken as a reference.
https://github.com/lua/lua/blob/75ea9ccbea7c4886f30da147fb67b693b2624c26/ltm.c#L43