Add a ValueParser

[ndmitchell]

Broken out from #7 and other discussions. Currently, values are "parsed" using UnpackValue, which for type T is fn(Value) -> Option<T>. That sucks because it doesn't say what you wanted or what didn't match. Imagine T is Vec<i32>, and the value is [1,"test"]. It should say the failure arises from "test" not being an Int at the 2nd element of the list. That means the return type should be a result with the error as Unexpected:

enum PathElement {
    Index(i32),
    Field(String),
}
struct Unexpected {
    path: Vec<PathElement>, // The path I followed, e.g. index 1
    expected: String, // What I wanted, e.g. Int
    got: Value, // What I got, e.g. "test"
}

We want this to be the ubiquitous method of getting data from inside Value, to ensure it's used everywhere, and thus we get good error messages always. That requires making it the simplest method and providing good support for this by default - we want manually unpacking a value to be more work.

That means the Unexpected type should be easy to construct, and the ValueParser trait should do it automatically for Vec etc.

We probably also want to be able to generate documentation from ValueParser saying what type the value expects in advance, to be used for auto-documentation and function signatures. That gives us approx:

trait ValueParser {
     fn parse(x: Value) -> Result<Self, Unexpected>;
     fn sig() -> String;
}