Desugar field constructs into their symbolic forms

[soareschen]

Summary

This PR updates the CGP proc macros to desugar field constructs into their symbolic forms, such as:

• symbol!("abc") is desugared to ι<'a', ι<'b', ι<'c', ε>>> instead of Char<'a', Char<'b', Char<'c', Nil>>>.
• Product![a, b, c] is desugard to π<a, π<b, π<c, ε>>> instead of Cons<a, Cons<b, Cons<c, Nil>>>

This change is aimed to simplify the inspection of expanded macro code, so that the desugared field constructs can be more readable inside the macro expansion.

Rename of symbol! to Symbol!

Furthermore, the use of symbol! are replaced with Symbol! (first letter capitalized). This is to align better with the case convention that the code generated from Symbol! is a type, not a value.

Nevertheless, the original symbol! macro is kept in the prelude as an alias to Symbol! for backward compatibility.