Lints to identify enums where variants have wildly different sizes

[nikomatsakis]

Because an enum takes the size of its largest member, it is very easy to have enums whose size grows surprisingly large if you are not careful. ast::Expr, for example, is currently 120 bytes in our compiler. It'd be helpful to have a lint that checks the size of the smallest and largest variant in an enum and reports if the difference is "too" big. What "too big" means I don't know -- perhaps if one is 2x the other?

One issue is what to do about generic enums. We could (for now) just assume that generics have a certain size, or else wait and report lint warnings on individual instantiations -- but this seems like a warning that should be associated with the def'n of the structure.

[nikomatsakis]

Another challenge: size information isn't really available until trans. So this'll have to be a kind of guesstimate. But that's probably ok.

[alexcrichton]

Similar to, but not the same as #6600

[huonw]

We could possibly have a post-trans lint phase.