Add `substream` and `stream` commands, `&seq` destructuring, and missing tests.

[okamsn]

• Add substream and stream commands.
  • Add stream.el to the dependencies.
  • Update Org documentation.
  • Add tests.
• Add &seq destructuring for values and places. The substreams should only be destructured using &seq.
  • Add tests.
  • Update Org documentation.
  • Define an error for destructuring substreams with &seq.
• Disable behavior of optional destructured values until we decide how that should work. See issue #198.
• Add missing array tests along with the new &seq tests.
  • Convert some tests to new macros loopy-def-pcase-test, loopy-def-pcase-test3, and loopy-def-loopy-ref-test.
• Use caching for loopy--get-var-groups.