refactor: Unify pk and content and timestamps into FIELDS

[maiha]

In master, when we want to iterate all fields, we have to process three kinds of fields pk, content columns and timestamps as following code. I feel this is a boring work and the code looks difficult.

def to_xxx
  # for PK
  {{ PRIMARY[:name] }}

  # for Content
  {% for name, type in FIELDS %}
     ...

  # for timestamps
  {% if SETTINGS[:timestamps] %}

This PR merges them into one FIELDS, and the primary and timestamp now becomes just wrapper macros to the field. Thus, we can simply iterate FIELDS to scan all fields.

def to_xxx
  {% for name, type in FIELDS %}
     ...

related refactor (Granite::ORM::Base)

• added CONTENT_FIELDS that removes PK from FIELDS
• added Base.fields to provide cached field names.
• added Base.content_fields to provide cached content field names.
• renamed Base#params to Base#content_values for clarity

Best regards,

[Blacksmoke16]

This would be quite helpful for the bulk-import PR :P

[faustinoaq]

Great, @maiha Thank you so much for all your contributions!!! 😄

[drujensen]

💯 🥇 Nice job. So much better!