Compute the size of a signed varint value.

def _SignedVarintSize(value):
  """Compute the size of a signed varint value."""
  if value < 0: return 10
  if value <= 0x7f: return 1
  if value <= 0x3fff: return 2
  if value <= 0x1fffff: return 3
  if value <= 0xfffffff: return 4
  if value <= 0x7ffffffff: return 5
  if value <= 0x3ffffffffff: return 6
  if value <= 0x1ffffffffffff: return 7
  if value <= 0xffffffffffffff: return 8
  if value <= 0x7fffffffffffffff: return 9
  return 10