A different approach to implementing alias.

The previous approach of PushAliasBuffer() only worked for a single
alias expansion.  Iteration was a problem.

Now we hook into ParseSimpleCommand and expand aliases iteratively.
We join the expanded aliases with the rest of the *text* from the
SimpleCommand, and then pass that into ParseCommand().

I made some comments about the cases that this doesn't handle.  And also
some notes about how dash and bash do it -- with global variables strewn
throughout the parser.

(Although I wonder if we could do that with self._Next() ? )

- Add more alias test cases
- Factor some free functions out of CommandParser.

All unit, spec, and gold tests pass.

core/lexer.py

@@ -38,6 +38,10 @@ def __init__(self, match_func, line, arena):

    self.Reset(line, -1, 0)  # Invalid line_id to start

  def __repr__(self):
    return '<LineLexer at pos %d of line %r (id = %d)>' % (
        self.line_pos, self.line, self.line_id)

  def Reset(self, line, line_id, line_pos):
    #assert line, repr(line)  # can't be empty or None
    self.line = line
@@ -216,6 +220,7 @@ def _ReadNormalInput(self, lex_mode):

  # TODO: Collapse newlines here instead of in the WordParser?
  def Read(self, lex_mode):
    # TODO: Remove this whole section
    while True:
      # Read from alias buffers first
      if self.buffers:
@@ -226,7 +231,6 @@ def Read(self, lex_mode):
        if t.id == Id.Eol_Tok:
          self.buffers.pop()
          continue  # read from next buffer or from the original line_Lexer

        # TODO: Translate tokens here?
        return t