declaring two locals with the same name in overlapping scopes is
a hard error.

<<<

main = fun () {
    var x int = 1
    var x int = 2
}

>>>

(unit
	(types
		(type 0 int primitive)
		(type 1 uint primitive)
		(type 2 bool primitive)
		(type 3 string primitive)
		(type 4 float primitive)
		(type 5 byte primitive)
		(type 6 ascii primitive)
		(type 7 void primitive))
	(functions
		(function 0 main main (returns (ref void)) (block
			(declaration x (ref int) (initializer (expr 1)))
		)))
	(emission_order 0 1 2 3 4 5 6 7)
	(diagnostics
		(error "name 'x' shadows an existing binding")))