Replace Rc<Vec<Val>> with Rc<[Val]> in interpreter

[samestep]

Currently the interpreter uses Rc<Vec<Val>> to hold arrays and tuples:

rose/crates/interp/src/lib.rs

Lines 20 to 21 in 15ac650

	Array(Rc<Vec<Val>>), // assume all indices are `Fin`
	Tuple(Rc<Vec<Val>>),

This is two layers of indirection rather than one; we should replace it with just Rc<[Val]>. The blocker is that ts-rs doesn't seem to currently support Rc<[T]>.