When available, use esraps's {def,in-}grammar in src/grammar.lisp

* src/grammar.lisp (defgrammar parser.ini): new grammar; for ini
  syntax
  (in-grammar parser.ini): new form; put following rule into the
  grammar
* src/protocol.lisp (parser string t): specify grammar class in
  `esrap:parse' call
* src/package.lisp (package parser.ini): added imported-from esrap
  symbols defgrammar and in-grammar, conditional on
  esrap.grammar-class feature
Use function terminals for variation points in src/grammar.lisp

* src/variables.lisp (toplevel): changed special
  declamation *assignment-expression* -> *assignment-operator*
  (*name-component-separator*): changed default value :. -> #\.;
  adapted documentation string
  (*assignment-operator*): renamed *assignment-expression*
  -> *assignment-operator*; adapted documentation
* src/grammar.lisp (defrule name-component-separator/.): adapted to
  changed variables
  (defrule name-component-separator/:): likewise
  (parse-name-component-separator/expression): new function; use value
  of `*name-component-separator*' to parse name component separator
  (defrule name-component-separator/expression): new rule; use
  `parse-name-component-separator/expression' when
  `*name-component-separator*' indicates that
  (defrule name-component-separator): added
  name-component-separator/expression
  (defrule name-component): adapted to renamed rules
  (assignment-operator/whitespace): renamed assignment/whitespace ->
  assignment-operator/whitespace; adapted to changed variables
  (assignment-operator/=): similar
  (assignment-operator/:): similar
  (parse-assignment-operator/expression): new function; use value of
  `*assignment-operator*' to parse assignment operator
  (defrule assignment-operator/expression): new rule; use
  `parse-assignment-operator/expression' when `*assignment-operator*'
  indicates that
  (defrule assignment-operator): added assignment-operator/expression;
  adapted to renamed rules
  (value-whitespace/newline): test whether
  `*value-terminating-whitespace-expression*' is #\Newline instead of
  :newline
  (defrule option): adapted to renamed rules
* src/package.lisp (package parser.ini): changed exported
  symbol *assignment-expression* -> *assignment-operator*
* test/grammar (test grammar.name-component-separator): likewise
* examples/etc.lisp: adapted to changes