Lay out the conformance code as common, v27 and v31r1

[jadamcrain]

Item 3 of #59.

3.1r1 imported 2.7's s8.helpers 48 times and placed definitions from 2.7's numbered packages. Now the code both revisions use is in common, 2.7's own code is in a flat v27, and 3.1r1 refers to the 2.7 definitions it reuses as v27.Procedure…, so each name says where it comes from.

Changes

• Procedure names. 2.7's procedure objects are named after their ids, with the dots replaced by underscores: Procedure8_2_1_2_1 (SBO, 0x28) and Procedure8_21_2_1 (double-bit read). Before, those two shared the name Procedure82121 in different packages, as did seven other pairs.
• v27/Tree.scala replaces the 57 Section.scala files. It's built with the section(id, title)(children*) builder that 3.1r1's tree used. The builder is now in api, replacing the varargs api.section.
• One-line procedure objects. The 80 one-line objects are folded into one file per 8.x section: Procedures8_2, 8_4, 8_5, 8_11, 8_14 and 8_22. Each is still its own top-level singleton. The other 77 procedures keep a file each.
• common holds:
  • the helpers from s8/helpers, except AttributeValidations and EventBufferSteps, which stay in v27
  • SupervisoryControl
  • the link-layer steps from s6's package object, as LinkSteps
• 6.6.2 and 6.6.3 helpers. These were members of those sections' Section objects and are now v27.PrimaryFrames and v27.SecondaryFrames.
• Visibility. private[helpers] is now private[common], and the numbered packages' private[sN] is now private[v27]. expectNoEventsUnsolicited is private[conformance], because both common and v27 use it.
• Tests moved to match: helper tests to common, the rest to v27. The procedure tests are renamed after their procedures, e.g. Procedure8_11_2_1Test.

Unchanged

Ids, titles, levels and order are the same. A dump compared before and after the move came out byte-identical. It covered:

• both revisions' trees
• every procedure's -describe output: options, steps and cleanup
• the disabled lists

It ran at each subset level of both revisions, in four configurations: the defaults, a device profile, every boolean option on, and every boolean option off. Each 3.1r1 placement still uses the same definition as before, now under its new name.