integrate the debugger into the repl

integrate the debugger into the repl

* module/system/repl/debug.scm: New file, defines a data type to hold
  state for a debugger stack, and some helper procedures to print the
  stack or print a frame. Most pieces are from (system vm debug).

* module/system/repl/error-handling.scm: New file, implements
  call-with-error-handling and with-error-handling, and instead of going
  into a debugger, we go into a recursive repl that happens to have
  debugging information. Will be removing the old debugger from (system
  vm debug) shortly.

* module/Makefile.am (SYSTEM_SOURCES): Add error-handling and debug scm
  files.

* module/system/repl/repl.scm (prompting-meta-read): Better error
  handling -- we don't want to go into a debugger when reading a
  command.
  (start-repl): Add #:debug keyword argument, and just dispatch to
  run-repl.
  (run-repl): New function, with the guts of the old start-repl. Added a
  prompt, to which a throw to 'quit will abort.

* module/system/repl/common.scm (repl-prepare-eval-thunk): New
  helper. In the future we will use this to not enter the debugger on
  errors that happen at compile time.
  (repl-eval): Use repl-prepare-eval-thunk.
  (repl-print): Run the before-print-hook when printing a value.

* module/system/repl/command.scm (*command-table*): Move `option' to the
  `system' group. Move `trace' to the `profile' group. Add `debug' and
  `inspect' groups.
  (command-abbrevs): Rename from command-abbrev, and allow multiple
  abbreviations.
  (display-group): Fix the case where abbrev? was #f.
  (display-summary): Fix alignment of the command and
  abbreviations. Allow multiple abbreviations.
  (read-command): Rename from read-datum, and have better error
  handling.
  (meta-command): Better error handling.
  (define-meta-command): Better error handling.
  (help, show, import, compile, disassemble, time, profile, trace): Fix
  docstrings and error messages.
  (define-stack-command): New helper, for commands that operate on a
  saved stack.
  (backtrace, up, down, frame, procedure, locals): New debugger
  commands, in the REPL now.
  (inspect, pretty-print): New "inspect" commands.