In spec https://github.com/toml-lang/toml#keys
Quoted keys are clarified as
> he exact same rules as either basic strings or literal strings
TOML clearly distinguishes basic string and multi-line basic string
(literal string is also).
https://github.com/toml-lang/toml#string
So table key and quoted key should not allow multi-line basic string
and multi-line literal string.
ABNF definition also describes that.
https://github.com/toml-lang/toml/blob/master/toml.abnf
```
string = ml-basic-string / basic-string / ml-literal-string / literal-string
quoted-key = basic-string / literal-string
```
`string` contains `ml-*` but `quoted-key` doesn't.
Turns out these are deserialized/serialized as tuples! While we're at it add
support for tuple variants and tuple structs through the same paths.
Closes#244
