Gracefully recover from incorrectly nested items

[estebank]

The parser currently only attempts to parse items and sub-items based on what the parent item is and pretty much gives up if, for example, a trait item is inside of an ADT, or an ADT is defined inside an ADT. The parser could try exhausting all the valid possible parses, and if it fails start trying to parse other cases that are invalid there and explain why it is not allowed and supply guidance on what to do instead. For example, a case taken from https://www.reddit.com/r/rust/comments/5tnjt8/question_regarding_nested_enum/:

enum Foo {
    enum Bar {
         Baz,
     },
    Bat,
}

gives

error: expected identifier, found keyword `enum`
 --> src/lib.rs:2:5
  |
2 |     enum Bar {
  |     ^^^^ expected identifier, found keyword
  |
help: you can escape reserved keywords to use them as identifiers
  |
2 |     r#enum Bar {
  |     ^^^^^^

error: missing comma
 --> src/lib.rs:2:9
  |
2 |     enum Bar {
  |         ^ help: missing comma

error: expected `:`, found `,`
 --> src/lib.rs:3:13
  |
3 |          Baz,
  |             ^ expected `:`

when instead it could give a single error saying enum definitions cannot be nested, or something to that effect.