Change Decimal's fields to ints.

[ghost]

Change Decimal's fields to ints.

https://devdiv.visualstudio.com/DevDiv/_workitems?id=447645

The code was ported from DWORD-based C++ and expects to
see unsigned values. Serialization expects to see signed fields.

Fixing this in three steps:

Iteration #1: Use VS autorename to rename the "hi/med/lo/flags" to
"uhi/umed/ulo/uflags"

Iteration #2: Manually revert the field rename (but not the references
to them) and change their types to signed int. Add back the
"u" versions as unsigned ints overlaying the signed versions
and invisible to serialization.

Iteration #3: Opportunistically change a few "u" references
back to the original mostly to get rid of obviously unnecessary casts
to uint.. Doing this very conservatively - only
where it's dead obvious without knowing all the intricacies of
uint vs. int math and and where it wouldn't create local mismatches
of "u" vs non-"u" references. Ugly as this looks, I don't think
converting the entire codebase to signed is in the cards.

Other approaches rejected:

• Implement a "ulo" property wrapping the "lo" field. Nope -
  some code passes refs to these fields.

• Implement a "private ref uint ulo => ref Unsafe.As<>..." property

  No-can-do - C# notices that the field belongs to a struct
  and won't let you pass around a ref to that so freely.

[morganbr]

Really nice approach on this, @atsushikan . Thanks for the fix!