Run the handler of SRFI-34's `with-exception-handler' in the right dyn. env.

Run the handler of SRFI-34's `with-exception-handler' in the right dyn. env.

* srfi/srfi-34.scm (with-exception-handler): Use `with-throw-handler'
  instead of `lazy-catch'.

* test-suite/tests/srfi-34.test ("SRFI 34")["`with-exception-handler'
  invokes HANDLER in THUNK's dynamic env."]: New test.

* test-suite/tests/srfi-39.test: Use `(srfi srfi-34)'.
  ("SRFI-39")["SRFI-34"]: New test.

* NEWS: Update.