On "const should be uppercase" lint, use structured suggestion

[estebank]

On the non_upper_case_globals lint, use span_suggestion to suggest the uppercase name.

We might want to consider applying the change everywhere the const is actually mentioned, which might be a big undertaking.

[bluetech]

I tried to implement just the simple variant of this (without the global rename). I have included the other nonstandard_style lints as well (snake case, camel case). Here is my attempt: https://github.com/bluetech/rust/commits/nonstandard-style-autofix

The problem I encountered is this: in order to make the suggestion applicable, the span needs to be accurate, i.e. it must span only the name/identifier itself. Currently it is not so in all cases. I (think) I have managed to make the spans accurate, except for 5 places, marked with XXX in the linked patch. They boil down to these cases:

• hir::Variant

• hir::intravisit::FnKind::ItemFn - strangely the Method variant has an Ident, but the ItemFn variant has only a Name.

• hir::Item - here is an example of an overlong span.

Is there a way to get a Span covering just the name in these cases?