Allow advanced lookups in lookup and lookup_mut

[kstrafe]

These commits allow one to search for tables with unconventional names. Example:

let table: toml::Value = r#"[table."name.other"] value = 0"#.parse().unwrap();
let look = table.lookup(r#"table."name.other".value"#).unwrap();

These commits have some breaking changes for the tests, as sending a raw " into lookup will no longer work correctly. One must send r#""\"""# or "\"\\\"\"" instead.

Performance: It may be possible to have a slight performance decrease for all lookups because each entry is now parsed and put into a String. This can be remedied by creating a lookup_simple method that takes simple names only.

Bugs discovered: the parser fails to parse "" and '' as a key name.

[alexcrichton]

Thanks! Can you add some negative tests for lookup as well for when parsing fails?

[alexcrichton]

I think there's a method to parse integers somewhere else, perhaps that could be leveraged here?

[kstrafe]

I think parsing integers accepts the +xx and -xx notation, which I don't think should be applicable. I'll need to look through the parser methods...
Edit: number_or_datetime can be used whilst evaluating the Value type to be an integer. I'll try leveraging it.
Edit2: fn integer may be the solution with allow_sign = false 👍

[kstrafe]

Added a bunch of negative tests and some positive ones.

I tried avoiding #[should_panic] as these tests easily break down if the toml is parsed wrong.

I'm a little uncertain of what convention to use in the tests.

[alexcrichton]

Looks great to me, thanks!