Experimental Rust Serialization Library.

This is an experiment to modernize rust's libserialize library. It is designed to implement https://github.com/rust-lang/rfcs/pull/22. rust-serde is an attempt to address a major shortcoming in libserialize. For normal structures, when you say you want to deserialize into:

struct Foo {
    x: int,
    y: int,
}

libserialize's deserializer essentially asks for:

• Is the next value a struct named "Foo"? If not, error.
• Is the next field named "x"? If not, error.
• Is the next value an "int"? If not, error.
• Is the next field named "y"? If not, error.
• Is the next value an "int"? If not, error.
• Is the struct finished? If not, error.

While this works for user defined structures, it cannot support deserializing into a value like json::Json, which is an enum that can represent every JSON value. In order to support that, it needs to be able to do some lookahead:

• What is the next value type?
  • If a struct, parse a struct.
  • If an integer, parse an integer.
  • ...

More formally, libserialize implements a LL(0) grammar, whereas json::Json requires a LL(1) grammar. rust-serde provides this by implementing a serializer and deserializer that produces a tagged token stream of values. This enables a Deserializable for json::Json to look at the next token before deciding on how to parse the value.

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There is now also a new library variation called serde2. This removes the need for tagged values and replaces them with a Visitor pattern. This pattern is very similar to the Iterator pattern, but it threads some custom state through visiting each type. This gets many of the benefits of the serde library without needing to always pay for tagging the variants.