Incorporate ice-9-debugger-extensions properly

Incorporate ice-9-debugger-extensions properly

i.e. put the extensions where they need to be, and delete
ice-9-debugger-extensions.scm.

* doc/ref/api-debug.texi (Single Stepping through a Procedure's Code):
  Change mentions of (ice-9 debugging ice-9-debugger-extensions)
  module to whatever is appropriate now (or just remove them).

* module/Makefile.am (NOCOMP_SOURCES): Remove
  ice-9-debugger-extensions.scm.

* module/ice-9/debugger.scm (debug-trap): Move here from
  ice-9-debugger-extensions.scm.

* module/ice-9/debugger/command-loop.scm ("continue", "finish",
  "step", "next"): Move here from ice-9-debugger-extensions.scm.

* module/ice-9/debugger/commands.scm (assert-continuable, continue,
  finish, step, next): Move here from ice-9-debugger-extensions.scm.

* module/ice-9/debugging/breakpoints.scm: Don't use
  ice-9-debugger-extensions module.

* module/ice-9/debugging/ice-9-debugger-extensions.scm: Removed.

* module/ice-9/debugging/trace.scm, module/ice-9/debugging/traps.scm:
  Remove more old version code.

* module/ice-9/debugging/traps.scm (guile-trap-features): Hardcoded as
  '(tweaking).