Add the initial draft for inference-using-bounds (#4140)

* Add the initial draft for inference-using-bounds

* Update the title line in feature-specification.md

* Make the diagrams top-down instead of left-to-right

That should improve the readability, since the new layout makes the
nodes bigger.

* Make the last diagram vertical as well

* Split the long lines into series of shorter lines

* Update the phrasing as suggested in the review comments

* Apply more of the phrasing adjustments suggested in the review comments

* Apply more phrasing suggestions

* Update the document version to 1.0 as suggested in the review comments

* Rename 'feature-specification.md' into 'design-document.md' as suggested in the review comments

* Mention the formal update in design-document.md

* Add a clarifying paragraph to explain a subgraph in a diagram

* Rename "Proposed changes" to "Rules" and make the language more definitive

* Add a clarifying paragraph and explanations for another diagram

* Include the change in grounded solution; adjust wording on the diagram

* Update the informal section describing the constraint solution for a set of type variables

* Update links in the first section

* Remove doubly embelished formatting; add minor corrections to the wording

* Be more consistent with using `<:` (is subtype of) and `<#` (is constrianed by)

* Make some updates to phrasing

* More updates to phrasing

* Apply the suggestions from the review comments