Editorial: Use Abstract Closure to set the code eval state

[jmdyck]

In GeneratorStart, AsyncGeneratorStart, and AsyncBlockStart, there is the following construct:

1. Set the code evaluation state of _context_ such that when evaluation
   is resumed for that execution context the following steps will be performed:
   1. <inner-steps>

This is a fairly novel construct, but we can reduce the novelty. Specifically, consider the <inner-steps>: they are to be performed later, but can refer to aliases defined in the outer algorithm. These are things that we see in Abstract Closures, so we can pull out those aspects of the construct:

1. Let _closure_ be a new Abstract Closure with no parameters
   that captures <aliases> and performs the following steps when called:
   1. <inner steps>
1. Set the code evaluation state of _context_ such that when evaluation
   is resumed for that execution context, _closure_ will be invoked.

(This has the side benefit of making the captures explicit.)

In this PR, the first commit makes the above change, and the second commit reduces the number of captured aliases by 'reconstructing' some of them within the closure.

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I wasn't sure what to do about the [fence-effects="user-code"] annotation, but I'm guessing that ecmarkup's handling of Abstract Closures takes care of it. (Another side benefit.)

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(There are also "Set the code eval state" steps in Await, GeneratorYield, and AsyncGeneratorYield, but I'm assuming that #2429 or #2665 will eliminate them.)

[syg]

lgtm, I like this use of ACs a lot.