[Yaml-core] Flow indentation

[Oren Ben-K. <or...@be...>]

Now that we have hopefully covered JSON compatibility :-) there is an 
open issue with regard to indenting flow content. The driving force is 
the following case:

  key: [
  ]

Which even appears as an (erroneous) example in the spec.

The simple thing is to allow flow content to be non-indented. This leads 
to all sort of problems, ranging from readability to breaking all type 
of heuristics (folding, error bounding, etc.). The worst is:

    ---
    foo: "string
    containing
    ...
    ---
    oops!"

Today, we _know_ that a ... line ends a document and a --- line starts a 
new one. Allowing flow content to be non-indented breaks that. YAML was 
designed to allow efficient streaming, and being able to break a stream 
into documents without parsing is important to us.

Here is an alternative. Today the rule is that all the flow content must 
be more-indented than the parent node. Instead, we can change the rule 
so that all scalar content must be more indented. The { [ , ] } 
indicators only need to be "as indented" as the parent node. This would 
mean:

    ---
    foo:
     bar: {
      "ok",
      ok
     } # ok
     baz: [
     "bad",
     bad
    ] # bad
    ...

This allows the intuitive placement of the end } ] where a C/JS 
programmer would expect it to be, while preserving all the methods 
based off indentation (folding, error bounding, document separation, 
etc.).

Thoughts?

Have fun,

	Oren Ben-Kiki