Improve precision for boxes

[momvart]

Boxes provide the simplest form of heap allocation in Rust. However, because they own a pointer, flowistry conservatively considers a maximum (unknown) region for them causing all of them to be grouped under a single alias group.
This behavior seems to be an exception, which can be rooted here.

flowistry/crates/flowistry/src/mir/utils.rs

Lines 613 to 614 in a156ab9

	_ if ty.is_box() => {
	self.visit_region(tcx.mk_region_from_kind(RegionKind::ReVar(UNKNOWN_REGION)));

As Boxes are probably the most-used structure for heap allocation, improving the precision of the analysis for them and avoiding reasoning about the pointer inside them can help a lot. It also affects the results for other types such as vectors when they use boxes. (e.g., using into_boxed_slice or some forms of vec! macro).