* 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'
