SI-7345 Extract a value class for the error modes within Context.

Soon to be extended to hold all the other Booleans in Context.
SI-7345 Represent as all boolean flags in Context in the ContextMode bitset.
SI-7345 Refactor lookup and setting of error reporting flags.
SI-7345 Refactor state propagation.

Less "copy these flags"; more "copy the lot".
SI-7345 Minor refactoring in Context#make.
SI-7345 Towards rationalizing the .make() overloads.
SI-7345 Inline the only-once-used newNamerFor.
SI-7345 Removing unused make() overloads.

And adding a TODO comment about unexplained behaviour.
SI-7345 Remove comment that appears obsolete.

Obsolete since ee02ad59c, as best as I can judge.
SI-7345 Use combinator to find next enclosing non-template.
SI-7345 Remove unused method isLocal().
SI-7345 Add Context#isLocal, akin to Symbol#isLocal
SI-7345 Encapsulate warning and error buffers in ReportBuffer.

  - merge `Context#{buffer, warningBuffer}` into `Context#reportBuffer`.
  - only expose immutable copies of the error and warnings buffers
  - Introduce a convenience method, `firstError`.
  - replace `condBufferFlush` more specific methods to retain or
    clear errors by error kind.
SI-7345 Exploit named/default args
  - Collapse overloads of `rootContext`
  - make `atOwner` more concise
SI-7345 More refactoring and documentation in Contexts

  - Use `firstError match {}` rather than `if (c.hasErrors) c.firstError ..`
  - Convert implementation comments to doc comments
  - Add new doc comments
  - Refactor `Context#make` for better readability.
  - Note a few TODOs.
  - Roughly delineate the internal structure of Contexts with comments.
SI-7345 Combine `mode` and `state` methods into `contextMode`.
SI-7345 Move `isNameInScope` closer to its compadres.
SI-7345 Decided to leave well enough alone with dead code warning forcing.

While `neg/check-dead.scala` indeed passes without this facility, I'm not
convinced that removing the force is a good idea. So instead I've removed the
TODO.
SI-7345 Restore alignment.

Whitespace only, nothing to see here.
SI-7345 Make a method in Contexts private.