Fix evaluation of escaped literals in double quotes.

For example, "\n" is untouched, but "\$" and "\"" are turned into single
characters.

This fixes 3 spec tests in quotes.test.sh.

Addresses issue #49.

core/id_kind.py

@@ -403,6 +403,9 @@ def _Dash(strs):
  return [(s, '-' + s) for s in strs]


# TODO: Need to check for binary -o first, before unary -o.
# Then you can fix unary -a too.

def _AddBoolKinds(spec):
  spec.AddBoolKind('BoolUnary', {
      OperandType.Str: _Dash(list(_UNARY_STR_CHARS)),

core/word_eval.py

@@ -851,7 +851,15 @@ def _EvalWordPart(self, part, quoted=False):
      val = part.token.val
      assert len(val) == 2, val  # e.g. \*
      assert val[0] == '\\'
      s = val[1]
      c = val[1]
      if quoted:
        # https://www.gnu.org/software/bash/manual/bash.html#Double-Quotes
        if c in ('$', '`', '"', '\\'):
          s = c
        else:
          s = val
      else:
        s = c
      return [runtime.StringPartValue(s, False, False)]

    elif part.tag == word_part_e.SingleQuotedPart:

test/spec.sh

@@ -229,9 +229,9 @@ subshell() {
    ${REF_SHELLS[@]} $OSH "$@" 
}

# Need to fix $ tokens, and $''
# Need to implement $'' escaping
quote() {
  sh-spec spec/quote.test.sh --osh-failures-allowed 4 \
  sh-spec spec/quote.test.sh --osh-failures-allowed 1 \
    ${REF_SHELLS[@]} $OSH "$@"
}