Introduce jomini::Value for loosely typed data

[nickbabcock]

🚧 👷 WIP code and WIP description👷 🚧

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This PR introduces jomini::Value, the analog serde_json::Value for jomini.

jomini::Value serves a variety of use cases.

untyped data

When reading data where you have no inkling of the structure of a field, and need to dynamically access and manipulate values.

Whereas with a TextTape, any edits to the underlying tokens (if such an API were to exist) risks corrupting object and array indices, jomini::Value is structured as a DOM tree, allowing for easier modifications.

It is not recommended to reach for jomini::Value when the structure of values can vary simply, as this use case is often covered by implementing Deserialize over an enum. For instance, the following data would not be a good use for jomini::Value

hello = "world"

# or

hello = { val = "world" }

as it can be represented by:

enum MaybeObject {
    Text(String)
    Object { val: String }
}

impl<'de> Deserialize<'de> for MaybeObject {
    // ...
}

lossless data

owned data

drawbacks

encoding issues

Performance

Write API

Example

We can write use jomini::Value to construct an eager variable interpolation program, which can then be deserialized into strongly typed object.

For instance, given:

@my_var = 10
my_obj = {
  pos_x = @[100-my_var]
  pos_y = @my_var
}

We deserialize it as:

my_obj = {
  pos_x = 90
  pos_y = 10
}

This is accomplished via the following steps:

• deserialize the input into a jomini::Value
• Make a pass over the value to collect all the variables
• Make another pass to interpolate values and update the underlying structure
• Finally we can either print out the resulting structure or deserialize a value from it.

TODO: code to follow