Define a clear order of operations for object element validation, remove "allowArbitraryKeys" option

[lucaswoj]

This PR refactors validateObject such that

• the order of precedence for object element validation logic is more intuitive and useful:
  1. a specific object element validator
  2. a specific object element specification
  3. a wildcard object element validator
  4. a wildcard object element specification
  5. error!
• the allowArbitraryKeys option is rolled into the above logic (case "v" ☝️)
• internal variable names are more consistent, giving better contextual clues to readers about how the function works

cc @lbud @jfirebaugh

[lucaswoj]

ref #559

[lbud]

@lucaswoj this looks good to me. (Should this just be re-targeted at mb-pages and we can close #559?)