Add Decimal32, Decimal64, Decimal128

[RaymondHuy]

Resolve #81376

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[huoyaoyuan]

If Int128 is not optimized, this can be optimally stored as a ReadOnlySpan of Int32/Int64 like Number.BigInteger.Pow10BigNumTable

[huoyaoyuan]

+0 and -0 should have the same hash code. The easiest way can be stripping the sign bit.

I also remember that there can be same values with different representations.

runtime/src/libraries/System.Private.CoreLib/src/System/Numerics/TotalOrderIeee754Comparer.cs

Lines 125 to 132 in 9b57a26

	else
	{
	// Equivalant values are compared by their exponent parts,
	// and the value with smaller exponent is considered closer to zero.
	// This only applies to IEEE 754 decimals. Consider to add support if decimals are added into .NET.
	return 0;
	}

[RaymondHuy]

@huoyaoyuan Thanks for your review. However this PR targets only the first phase as following: #81376 (comment) so I haven't added NegativeZero and PositiveZero. I will update GetHashCode method in the next phase.

[tannergooding]

This is on my radar to take a look at; just noting it might be a bit delayed due to other priorities for the .NET 9 release.

CC. @jeffhandley

[marchewek]

sorry for being picky - the last two commits have titles that don't really match the contents, it could be improved (for traceability's sake)

[RaymondHuy]

Hi @tannergooding, I'm just curious when this PR can be reviewed ? 😄

[huoyaoyuan]

Maybe after feature complete of .NET 9. As the original API review, this API is out of scope for .NET 9, as well as BFloat16. Now we are close to final stage of .NET 9 and have really no time to include this.

[tannergooding]

As huoyaoyuan indicated I would expect to get to this in early .NET 10 at this point in time (there's only around 10 more days before the next snap, at which point we start locking down for RC1).

I really appreciate the effort put into this and the drive to try and get it in, it just wasn't part of the "scheduled" work for .NET 9 and so continued to be deprioritized in comparison to the other feature work going on in the numerics space (mostly due to the size/complexity involved here).

I do still intend to drive this in and will do my best to ensure this is included as part of the scheduled work for .NET 10.