DOI 2.22.23 Add metaspec builder functions

[reweeden]

These helper functions make it simpler to create the metaspec file by encapsulating some of the boilerplate JSON structure and adding ability to use python operators to generate arithmetic directives.

TEMPLATE = build({
    "key": mapped("source", "some.key") + mapped("source", "some.other.key")
})

Is equivalent to writing

TEMPLATE = {
    "key": {
        "@add": {
            "left": {
                "@mapped": {
                    "source": "source",
                    "key": "some.key",
                },
            },
            "right": {
                "@mapped": {
                    "source": "source",
                    "key": "some.other.key",
                },
            },
        },
    },
}