Implement <&, with test.

Fixes #20.

core/process.py

@@ -123,11 +123,14 @@ def _ApplyRedirect(self, r, waiter):
      self._PushClose(target_fd)

    elif r.tag == redirect_e.DescRedirect:
      if r.op_id == Id.Redir_GreatAnd:  # 1>&
      if r.op_id == Id.Redir_GreatAnd:  # 1>&2
        if not self._PushDup(r.target_fd, r.fd):
          ok = False
      elif r.op_id == Id.Redir_LessAnd:  # 0<&5
        # The only difference between >& and <& is the default file
        # descriptor argument.
        if not self._PushDup(r.target_fd, r.fd):
          ok = False
      elif r.op_id == Id.Redir_LessAnd:
        raise NotImplementedError
      else:
        raise NotImplementedError

spec/redirect.test.sh

@@ -1,5 +1,17 @@
#!/usr/bin/env bash

### >&
echo hi 1>&2
# stderr: hi

### <&
# Is there a simpler test case for this?
echo foo > $TMP/lessamp.txt
exec 5< $TMP/lessamp.txt
read line <&5
echo "[$line]"
# stdout: [foo]

### Leading redirect
echo hello >$TMP/hello.txt  # temporary fix
<$TMP/hello.txt cat