Handle backslash escapes at parse time rather than at runtime.

Addresses issue #49.

core/word_eval.py

@@ -851,17 +851,7 @@ def _EvalWordPart(self, part, quoted=False):
      val = part.token.val
      assert len(val) == 2, val  # e.g. \*
      assert val[0] == '\\'
      c = val[1]
      # TODO: This can be done at compile time instead!  Change _BACKSLASH
      # definition in DQ state.
      if quoted:
        # https://www.gnu.org/software/bash/manual/bash.html#Double-Quotes
        if c in ('$', '`', '"', '\\'):
          s = c
        else:
          s = val
      else:
        s = c
      s = val[1]
      return [runtime.StringPartValue(s, False, False)]

    elif part.tag == word_part_e.SingleQuotedPart:

osh/lex.py

@@ -282,7 +282,12 @@ def IsKeyword(name):
  C('|', Id.Lit_Chars),
] + _UNQUOTED

LEXER_DEF[LexMode.DQ] = _BACKSLASH + _LEFT_SUBS + _VARS + [
LEXER_DEF[LexMode.DQ] = [
  # Only 4 characters are backslash escaped inside "".
  # https://www.gnu.org/software/bash/manual/bash.html#Double-Quotes
  R(r'\\[$`"\\]', Id.Lit_EscapedChar),
  C('\\\n', Id.Ignored_LineCont),
] + _LEFT_SUBS + _VARS + [
  R(r'[^$`"\0\\]+', Id.Lit_Chars),  # matches a line at most
  # NOTE: When parsing here doc line, this token doesn't end it.
  C('"', Id.Right_DoubleQuote),