Suggest to store large enum variants as Box

[stanislav-tkach]

For example:

enum Test {
    A(i32),
    B([i32; 8000]),
}

In this case it probably would be better to use B(Box<[i32; 8000]>).

[Manishearth]

Tip for anyone trying to solve this bug: https://github.com/Manishearth/rust-clippy/blob/04c8108e3f473dbe80b8e67e18e923c91c362b5e/clippy_lints/src/escape.rs#L194 contains some code that extracts the size of a type. Probably should extract that into utils, and use it.