Properly implement break/continue/return.

The existing implementation was buggy, as exposed by the tests I added
in loop.test.sh and func.test.sh.

I removed break/continue/return as builtins, and I now parse them
**statically**.  This will break some overly clever programs like:

    b=break;
    for i in a b c; done
      echo $i
      $b
    done

But I'm being consistent with statically-parsed assignment, and it may
make it easier to convert to Oil and do other program analysis.

Representation (i.e. the thin waist of the interpreter):

- core/id_kind.py: Add Id for break/return/continue, under new Kind
  ControlFlow
- osh.asdl: add a ControlFlow variant to the command type

Algorithm:

- osh/lex.py: the lexer will return the new Ids
- osh/word_parse.py: ControlFlow tokens are treated as literals here, like
  Kind.KW, Kind.Assign, Kind.BoolUnary, etc.
- osh/cmd_parse.py: parse words consisting of only ControlFlow tokens
  into a ControlFlow node.
  - Consistently catch errors like redirects and env prefixes on
    non-commands like ControlFlow and Assign.  Tested in
    command-parsing.test.sh.
- cmd/cmd_exec.py:
  - Introduce a new _ControlFlow exception.  Get rid of the cflow return
    value.
  - Loops catch break and continue; functions catch return
  - Check some errors dynamically.  Print an error when
    break/continue/return bubble up to the top level.  The error message
    should be improved.

Tests:

- Rename func -> func-parsing, and func test
- Misc tweaks, e.g. fix a test that wasn't checking the error status.
- Bump down --osh-allowed-failures because the fix to print to stderr in
  ui.PrintError() made several of them pass
- Fix bug where we were capped at 40 tests.  Now 528 total cases, OSH
  passing 305, OSH failing 165.

Other cleanup:

- Get rid of ExecuteTop function.

bin/oil.py

@@ -97,10 +97,10 @@ def InteractiveLoop(opts, ex, c_parser, w_parser, line_reader):
      if ast_f:
        ast.PrettyPrint(node)

      status, cflow = ex.ExecuteTop(node)
      status = ex.Execute(node)

      if opts.print_status:
        print('STATUS', repr(status), cflow)
        print('STATUS', repr(status))

    # Reset prompt and clear memory.  TODO: If there are any function
    # definitions ANYWHERE in the node, you should not clear the underlying
@@ -231,9 +231,9 @@ def OshMain(argv):
      ui.PrintError(err, arena, sys.stderr)
      return 2  # parse error is code 2

    status, cflow = ex.Execute(rc_node)
    status = ex.Execute(rc_node)
    #print('oilrc:', status, cflow, file=sys.stderr)
    # Ignore bad status?  What about cflow?
    # Ignore bad status?
  except IOError as e:
    if e.errno != errno.ENOENT:
      raise
@@ -324,7 +324,7 @@ def OshMain(argv):
        ast_f.write('\n')

    if opts.do_exec:
      status, cflow = ex.Execute(node)
      status = ex.Execute(node)
    else:
      util.log('Execution skipped because --no-exec was passed')
      status = 0

core/builtin.py

@@ -110,8 +110,7 @@
# - So you can just add "complete" and have it work.

EBuiltin = util.Enum('EBuiltin', """
NONE BREAK CONTINUE RETURN READ ECHO EXIT SOURCE DOT TRAP EVAL EXEC SET
COMPLETE COMPGEN DEBUG_LINE
NONE READ ECHO EXIT SOURCE DOT TRAP EVAL EXEC SET COMPLETE COMPGEN DEBUG_LINE
""".split())


@@ -120,10 +119,13 @@
# On the other hand, 'cd' CAN be redefined.
# TODO:
# - use these
# - local and declare should be here, since export and readonly are.
SPECIAL_BUILTINS = [
    'break', ':', 'continue', '.', 'eval', 'exec', 'exit', 'export',
    'readonly', 'return', 'set', 'shift', 'times', 'trap', 'unset',

    # local and declare are not POSIX, but should be here since export and
    # readonly are.
    'local', 'declare',
]


@@ -309,14 +311,7 @@ def Resolve(self, argv0):
    # For completion, this is a flat list of names.  Although coloring them
    # would be nice.

    if argv0 == "break":
      return EBuiltin.BREAK
    elif argv0 == "continue":
      return EBuiltin.CONTINUE
    elif argv0 == "return":
      return EBuiltin.RETURN

    elif argv0 == "read":
    if argv0 == "read":
      return EBuiltin.READ
    elif argv0 == "echo":
      return EBuiltin.ECHO