Update iteration commands

[okamsn]

This PR addresses #65.

WORK IN PROGRESS

• Add macro to simplify definitions, similar in use to loopy-defaccumulation.
  • Add support for optional arguments and keyword arguments.
• Add distributing elements of multiple sequences.
  • TODO: What is the best data type for iterating through sequences? We can
    choose what to make the sequence resulting from distribution.
• Try to add equivalent keywords for Iterate's sequence drivers.
• Make command arguments more regular. For example, I haven't checked
  distributing elements for commands like list-ref, but it's still a good idea
  to change its optional FUNC argument to :by, to be more like the command
  list, for which an optional FUNC argument is no longer possible.

[okamsn]

Current commit message:

• Add heuristic to determine whether we should quote a list.
• Create macro loopy--defiteration, similar in use to
  loopy--defaccumulation.
• Add multi-sequence support to list, array, and seq commands.
• Add macros loopy--distribute-sequence-elements,
  loopy--distribute-array-elements, and loopy--distribute-list-elements.
• Add test to check for ELC files in the current working directory. These files
  can accidentally be used instead of the EL files, meaning that we won't see our
  changes during testing.
• Add the keywords from, upfrom, downfrom, to, upto, downto,
  above, below, and by to commands seq and array, the -ref variants,
  the -index variant, and the nums command and variants. Add :by to
  cons and list and variants.
  • For nums, keep the positional arguments, but make them optional.
• Add to library comments a note on how to find loop commands with Imenu.
  This is helpful now that we are using a macro to define them.
• In code, add more headings for outline commands.
• Add macro loopy--plist-bind. This macro is similar to
  cl-destructuring-bind, but for property lists.
• Add function loopy--find-start-by-end-dir-vals to extract the needed values
  from plists. This function is used for all of commands which share so many
  keyword arguments.
• Split "Iteration" section of Org doc into several sub-sections.
  • Break down into sections.
  • Add descriptions and custom ID's for sections.
  • Re-arrange commands.
• Add a changelog.

[okamsn]

I think these changes are ready. They can also be viewed in the multi-seq branch.

User @Luis-Henriquez-Perez, is there anything that you think should be changed?