Cuttlebone 0.15.0

Milestone 15: what a client that indexes Inko itself still needed.
Report.

• Quick fixes in files with other errors: a fix is offered when it
  removes its diagnostic and adds no error, not only when the file then
  has none. Imports are offered for the names in the requested range,
  also for a client that sends no diagnostics in the context.
• Four more quick fixes: missing-field (the field added with a
  value of its type), invalid-implementation (every method the impl
  block lacks, stubbed, in one action for all its diagnostics),
  missing-trait (impl R for T with its methods), unreachable (the
  code removed). Each still passes inko check.
• Completion of names in code being edited is refused only where a
  change or an error can bind or hide a name, not whenever any
  declaration differs; and keywords, where the grammar takes them.
• Document symbols hold still: a brace a method's body opens and
  nothing closes yet leaves out only that method, not the type it's in.
• Signature help for a type's constructor (Point(x: 1)), from its
  fields.
• Folding ranges' collapsedText, for clients that take it.
• Workspace symbols for an index: impl blocks, every symbol for an
  empty query, and every match of any query as partial results.
• For Inko 0.21.1, as before. No patch change, no new extension.

Each archive works with one Inko version, which its name says.
Machine-readable list of the builds, with sizes and SHA-256:
https://github.com/setdef/Cuttlebone/releases/download/v0.15.0/manifest.json
Every release, with its Inko versions and manifest:
https://github.com/setdef/Cuttlebone/releases/latest/download/index.json
(docs/releases.md).