Implement StaticString trait to produce &'static str from symbol types

[soareschen]

Summary

This PR introduces the StaticString trait, which makes it possible to produce a &'static str value from a generic symbol type:

pub trait StaticString {
    const VALUE: &'static str;
}

As an example, the type Symbol!("abc") would implement StaticString with "abc" as VALUE.

Symbol Type

The &'static str value is produced as a const through const evaluation. To support the const creation of strings, the Symbol! macro has been modified to include the byte length of the string inside the expansion. For example, Symbol!("abc") would be expanded into:

Symbol<3, Char<'a', Char<'b', Char<'c', Nil>>>>

The Symbol type is defined as:

pub struct Symbol<const LEN: usize, Chars>(pub PhantomData<Chars>);

which basically attaches the byte length of the string as metadata. This is needed for now, as it is not possible to make use of the calculation of the byte length through const generics evaluation inside a type expression like [u8; T::LEN]. On the other hand, by including the LEN explicitly, we can avoid doing any const evaluation and use that to produce [u8; LEN].

ψ and ζ Aliases

The new Symbol type has ψ as its alias, and the alias for Char has been changed from ι to ζ. So with this, the type Symbol!("abc") is shown as the following inside macro expansion:

ψ<3, ζ<'a', ζ<'b', ζ<'c', ε>>>>

The reason we renamed ι to ζ for Char is because ι will trigger the confusable_idents warning in Rust if the user code contains the use of i as an identifier.