Context typing with some special methods like `#freeze`

[soutaro]

Problem

We often write code using #freeze or #-@.

# @type const DEFAULT_KEY: "user_id"
DEFAULT_KEY = "user_id".freeze

This doesn't work because literal type cannot be given to non-literal expressions.

Proposal

Let Steep to infer literal types on a method call when the following conditions hold:

1. The method returns self
2. The receiver can have literal type

The rule No. 1 would be the key. It means that the method says the type of the return value is exactly same with the type of the receiver.

[rainhead]

Ran into this issue with a frozen record. Took a while to figure out where the error was coming from.

[soutaro]

Cannot be done in Steep 1.5.