…or workgroup variable

* Refines "fixed-footprint" to add subcategory of "creation-fixed footprint",
  to mean a plain type whose memory footprint is fixed at shader
  creation time. The "fixed-footprint" remains defined as a plain type
  whose memory footprint is fixed by the pipline-creation time.

* Allow element count on a fixed-size array to be an overridable
  module-scope constant.
  * Later I expect we'll have a proper category and name for pipeline-constexpr
    and we can patch this up at that time.

* Refine the rules about when two array types are the same.

* Refine the rule about initial value of a workgroup variable.

* Previously "fixed-size array" meant element count was a
  shader-creatio-time constant. Now it allows pipeline-creation
  constant, so revisit all uses of "fixed-size array" and further
  qualify with "with creation-fixed footprint" in the intended places:

  In particular:
   - array element must be fixed-footprint.
   - structure member that is fixed-size array must be fixed-footprint

* Note and provide examples that the store type for a workgroup variable
   may have an element count that is pipeline-overridable.
   It has fixed footprint but not necessarily creation-time fixed footprint.

* Editorially emphasize that type constructors only create constructible
  types.

Fixes: gpuweb#2024