`msg.to` builtin for global access to `address(this)`

Please add the `msg.to` property. This property is simply the address of the currently executing contract. It is the value of `address(this)`.

Using `msg.to` makes sense in free functions. `address(this)` is not currently available in free functions.

Some people may argue that `address(this)` can be passed as an argument to free functions. My counter argument is that we don't have to pass `msg.sender` or `msg.data` or `msg.value` as arguments, why do we have to pass `address(this)` as an argument?

It is fine if people want to pass `address(this)` as an argument. However, I think people should have the option to just be able to use `msg.to`. I would certainly use that.

`address(this`) can coexist with `msg.to`.

Start by reviewing how the existing msg.sender, msg.data, and msg.value builtins are defined and how free functions resolve execution context. Confirm the intended semantics of msg.to as address(this), including its availability in free functions; done means the language accepts and evaluates the new property with coverage for the behavior.

Written by the indexing model from the issue text.