Refactor Field type to have class path and field name as const parameters

[giovanniberti]

This would change Field definition from

struct Field<'env: 'borrow, 'borrow, T> { /* ... */ }

to

struct Field<'env: 'borrow, 'borrow, const CLASSPATH_PATH: &str, const FIELD_NAME: &str, T> { /* ... */ }

This change would allow:

1. More type safety around fields (you could not directly assign a class field where another class field is expected), even though I don't know how much useful this kind of safety would be
2. Removal of the #[field] attribute in *FromJavaValue derive macros for Field fields

Moreover, in *FromJavaValue derive macros we could still ensure that the two constant parameters match the declared package and class name.

With some macro trickery we could perhaps include a "pseudo type macro" Field! that expands to the correct Field<...> type and does not actually exist but is instead manually expanded by the derive macros.

Blocked by rust-lang/rust#44580