Fixes #500, since all (known) roundtrip issues are now documented and tested. Once serde allows us to fix them, they can be removed from the newly expanded restrictions section.

• externally tagged enums
• internally tagged enums
• internally tagged structs
• adjacently tagged enums
• untagged enums
• other variant in internally tagged enums
• other variant in adjacently tagged enums
• serialize flattened struct
• serialize flattened struct variant
• deserialize flattened struct
• deserialize flattened struct variant
  • externally tagged
  • untagged
  • adjacently tagged
  • internally tagged

• document one-tuple/one-array limitation inside unwrapped newtype variants
• document that serde only supports unit, bool, integer, float, char, string, and byte keys inside flattened structs, and only string and byte keys and u8 and u64 when the flattened struct is inside an untagged or internally tagged enum document that we only support string keys inside flattened maps (even though serde technically permits more, this support is not roundtrip-safe as soon as flattened maps are involved)
• document that serde does not support flattened structs containing conflicting keys, e.g. an inner key matches a later outer key or two flattened maps in the same struct share a key
• document that #[enable(implicit_some)] is not supported with Options inside #[serde(flatten)]
• document that PrettyConfig::struct_names(true) is not supported inside flattened structs / struct variants

• Add tests for any bug fixes
• Add tests for all known bugs
  • structs or struct variants that contain a #[serde(flatten)]ed field:
    • are only serialised as maps and deserialised from maps
    • must not contain duplicate fields / keys, e.g. where an inner-struct field matches an outer-struct or inner-struct field
    • must not contain more than one (within the super-struct of all flattened structs) #[serde(flatten)]ed map field, which collects all unknown fields
    • if they contain a #[serde(flatten)]ed map, they must not contain:
      • a struct that is not flattened itself but contains some flattened fields and is flattened into the outer struct (variant)
      • an untagged struct variant that contains some flattened fields
      • a flattened externally tagged newtype, tuple, or struct variant, flattened internally tagged unit, newtype, or struct variant, or any flattened adjacently tagged variant
      • a flattened tagged struct
  • ron only supports string keys inside maps flattened into structs
  • internally (or adjacently) tagged or untagged enum variants or #[serde(flatten)]ed fields must not contain:
    • i128 or u128 values
    • struct names, e.g. by enabling the PrettyConfig::struct_names setting
    • newtypes
    • zero-length arrays / tuples / tuple structs / structs / tuple variants / struct variants
      • Options with #[enable(implicit_some)] must not contain any of these or a unit, unit struct, or an untagged unit variant
    • externally tagged tuple variants with just one field (that are not newtype variants)
    • tuples or arrays with just one element are not supported inside newtype variants with #[enable(unwrap_variant_newtypes)]
    • a ron::value::RawValue
  • untagged tuple / struct variants with no fields are not supported
  • untagged tuple variants with just one field (that are not newtype variants) are not supported when the #![enable(unwrap_variant_newtypes)] extension is enabled
  • internally tagged newtype variants and #[serde(flatten)]ed fields must not contain:
    • a unit or a unit struct inside an untagged newtype variant
    • an untagged unit variant
  • serializing a ron::value::RawValue using a PrettyConfig may add leading and trailing whitespace and comments, which the ron::value::RawValue absorbs upon deserialization
• I've included my change in CHANGELOG.md
• Check all // BUG: comments in the fuzzer again to ensure that nothing was missed / is unnecessary or out of place