ub left shifting negative signed values

[nmm0]

Describe the bug
Some bitfields trigger undefined behavior by left-shifting a negative value.

To Reproduce
Adding this assert to setFieldDynamic in bits_packer.impl.h will reliably fail in our tests:

  auto tmp = ((static_cast<BitField>(segment) << nbits) >> nbits);
  if ( std::is_signed< decltype(tmp) >::value && ( tmp < 0 ) )
    vtAbort(fmt::format(
      "hit a negative value of {}, segment: {}, nbits: {}", tmp,
      static_cast<BitField>(segment), nbits ) );

The next line is

field = field | (((static_cast<BitField>(segment) << nbits) >> nbits) << start);

So this issue being that the tmp value here is negative (BitField is signed in some cases) and being left shifted by start, which is undefined behavior.

Expected behavior
we need to make sure the shifted value is unsigned.