Convert guile exceptions to R6RS conditions in R6RS exception handlers.

Convert guile exceptions to R6RS conditions in R6RS exception handlers.

* module/rnrs/exceptions.scm (&guile): New condition type.

  (guile-condition-converters): New variable.

  (convert-guile-condition, default-guile-condition-converter,
  set-guile-condition-converter!, guile-common-conditions,
  guile-lexical-violation-converter, guile-syntax-violation-converter,
  guile-assertion-violation-converter, guile-system-error-converter,
  guile-undefined-violation-converter, guile-error-converter,
  guile-implementation-restriction-converter): New procedures.

  (with-exception-handler): Catch all exceptions, not just R6RS
  exceptions.  Convert native Guile exceptions to R6RS conditions,
  preserving the original Guile exception information in the &guile
  condition object.

  (raise): If the condition includes a &guile condition, use 'throw' to
  throw the original native guile exception instead of raising an R6RS
  exception.

* test-suite/tests/r6rs-exceptions.test ("guile condition conversions"):
  Add tests.