Some tweaks to the R6RS support

Some tweaks to the R6RS support

* module/rnrs/base.scm (error, assert): Define -- they were missing.
  (assertion-violation): Properly treat a #f `who' argument.

* module/rnrs/conditions.scm (condition): Use `assertion-violation'
  instead of the undefined `raise'.
  (define-condition-type): Fix for multiple fields.
* test-suite/tests/r6rs-conditions.test: Test accessors of a
  multiple-field condition.  Also import `(rnrs base)' to allow
  stand-alone running of the tests; apparently the `@' references
  scattered throughout the R6RS modules make the libraries sensitive to
  their load order -- for instance, trying to load `(rnrs conditions)'
  before `(rnrs base)' is loaded fails.

* module/rnrs/records/inspection.scm: Use `assertion-violation' instead
  of an explicit `raise'.
* module/rnrs/records/syntactic.scm (process-fields): Use
  `syntax-violation' instead of bogus invocations of `error'.