allow memo on methods

[ch1ffa]

A new version of memo macro that allows us to apply it on methods as follows:

• if argument db is provided, e.g. #[memo(db = my_db) then we try to use this argument as a database. Unfortunately we don't know a concrete Database implementation so we can find it only by name
• if there is no db macro argument and the first argument is &self we try to use it as database
• if memoized function is a free function, then we use its first argument as database

one can use #[memo(db)] if name of the argument is db, #[memo(db = test_db)] or #[memo(db = "test_db")]

[rbalicki2]

Overall looks great! Just some minor nits.

[rbalicki2]

YES

[rbalicki2]

nifty

[rbalicki2]

what if we access self here? That would break memoization, right?

I suppose that's true of methods on structs that implements Database, but maybe then it's clear that you are only supposed to store sources on that struct...

[rbalicki2]

should we disallow &mut self in the macro? It looks like we rewrite it to be &self regardless, which makes for confusing errors:

    #[memo]
    fn first_letter_2(&mut self, input_id: SourceId<Input>) -> char {
        self.storage.set(Input {
            key: todo!(),
            value: todo!(),
        });
       

      #[memo]
|     ^^^^^^^ `__db` is a `&` reference, so the data it refers to cannot be borrowed as mutable