* module/ice-9/peg.scm: rename 'peg-sexp-compile' to
'compile-peg-pattern'
* module/ice-9/peg/codegen.scm: same
* module/ice-9/peg/string-peg.scm: same
* module/ice-9/peg/using-parsers.scm: same
* doc/ref/api-peg.texi: same
module/ice-9/peg/string-peg.scm: update S-expression generators to use the
new *, +, ?, followed-by, and not-followed-by forms.
module/ice-9/peg/codegen.scm: remove the 'body' form in the PEG s-expression
representation.
* module/ice-9/peg/codegen.scm: Make the PEG syntax extensible, and
move most of the current code generators to the new interface
* doc/ref/api-peg.texi: Document PEG extensions in the PEG Internals
section of the manual
* module/ice-9/peg/codegen.scm: peg-sexp-compile no longer knows about
string PEGs
* module/ice-9/peg.scm: add a new function peg-extended-compile that
calls peg-sexp-compile or peg-string-compile on its argument as
appropriate
* module/ice-9/peg/string-peg.scm: remove define-nonterm and make a simpler
macro called `define-sexp-parser' to make the PEG grammar
* module/ice-9/peg.scm: move define-nonterm macro to this file
* module/ice-9/peg/codegen.scm: move code to wrap a parser result nicely to
this file, under name `wrap-parser-for-users'
