Support as and with

[cohix]

TL;DR Variable names and named function args!

This adds support for the as and with clauses in a Directive handler's steps. For example:

steps:
      - group:
        - fn: modify-url
        - fn: helloworld-rs
          as: hello
      - fn: fetch-test
        with:
          - "url: modify-url"
          - "logme: hello"

This allows the developer to do two things:

• Define the key used to store the result of a function (i.e. helloworld-rs becomes hello in state). This is analogous to "setting a variable"
• Define a custom state to be passed into a function (i.e. fetch-test gets keys url and logme instead of the "naked" state of modify-url and hello. This is analogous to passing args into a function, since the developer can't expect a function to take args with the names of their other functions.