Improve dependency tracking for dead code elimination

[RyanGlScott]

Currently, dependency tracking for mir-json-dce's dead code elimination is implemented in a very hacky way. One of the more obvious symptoms is here:

mir-json/src/lib_util.rs

Lines 190 to 195 in 5d697d5

	JsonValue::String(ref s) => {
	if s.contains("::") {
	let id2 = self.intern.intern(s.into());
	self.dep_map.entry(id).or_insert_with(HashSet::new).insert(id2);
	}
	},

The idea is that only names that contain :: are eligible for dependency tracking, which is why we check for it here. For instance if we see {"name": "foo::bar"} in one place and {"kind": "call", "fn": "foo::bar"} in another, then the second depends on the first. This check also filters out names like "call" in that example, which we do not want to track.

The downside, of course, is that you have to ensure that all names that you care about actually do contain ::, or else unexpected things will happen. In my case, it caused the name alloc$0 to not appear at all in a key part of the JSON!

We should reimplement dependency tracking in a way that avoids the need for this hack.