# Support table as first argument to newproxy

## Summary

Introduce a new way of creating `userdata` objects from Luau with an existing metatable.

## Motivation

Since Luau is derived from Lua 5.1 it retains the `newproxy` function used to create custom userdata objects from Lua. While this function is used by a number of libraries it has its limitations.

The `newproxy` function accepts a single, boolean parameter which specifies if the newly created userdata should be initialized with a metatable or not. If the userdata is initialized with a metatable then it can be retrieved and edited by calling `getmetatable`. Unfortunately, this means that a userdata created through `newproxy` cannot share a metatable with another userdata. There are two main implications of this:
- Memory - A userdata with a metatable has the same cost as a table with a metatable.
- Usability - userdata with the same 'type' do not have the same metatable.

## Design

Allow `newproxy` to be called with a table or nil instead of a boolean.
- When it is called with a table, the newly constructed userdata will have that table assigned as its metatable.
- When it is called with nil, the newly constructed userdata will have no metatable.

Passing a boolean to `newproxy` will continue to be supported for backwards compatability but is not recommended over the proposed behavior.

## Drawbacks

TODO: It will increase the usage of newproxy which may not be desirable.

## Alternatives

TODO: New method entirely - add more support for userdata