Refactor the character set abstraction:

Refactor the character set abstraction:

* Clarify the use of "code point" versus "scalar value".

* Rename well-formed-scalar-value-list? to code-point-list? and broaden its
  scope to allow characters, strings, and character sets.

* Rename scalar-values->char-set to char-set* and broaden its domain to include
  any code-point-list?.

* Rename char-set->scalar-values to char-set->code-points.

* Implement char-in-set? which is char-member? with the args reversed.  This
  makes it consistent with scalar-value-in-char-set?.  Deprecate char-member?.

* Implement char-set-union* and char-set-intersection*.

* Eliminate all of the "alphabet" names which are obsolete.

* Eliminate guarantee-char-set and error:not-char-set.