Support for common stanzas

[Nexmean]

Cabal has feature called common stanzas. It described here: https://vrom911.github.io/blog/common-stanzas. Can we add support for this feature to hpack?

[tfausak]

hpack already supports this: https://github.com/sol/hpack/blob/f03825c/README.md#not-repeating-yourself

The blog post you shared discusses this as well: https://vrom911.github.io/blog/common-stanzas#hpack

[brandonchinn178]

It would be nice to get first class support for this. For example, if I want to define a common stanza that adds foo to dependencies, I can't do it today because:

1. Using <<: *add-foo will add a dependencies key, but itll get overridden when defining dependencies a second time in the same block
2. YAML doesnt support concatenating lists, so I cant do something like

dependencies:
    <<: *add-foo
    - base

[ramirez7]

FYI I've created a new issue for this here #569 instead of necro-posting on this issue 😁