Add span-based methods to Decimal

[stephentoub]

Fixes #28904
cc: @tannergooding, @GrabYourPitchforks

[GrabYourPitchforks]

Some comments on tests primarily, otherwise LGTM.

[GrabYourPitchforks]

[ahsonkhan]

fyi @carlossanlop, @maryamariyan - label for "new api needs doc" missing.

[ahsonkhan]

Otherwise, looks good.

[ahsonkhan]

There are a few other places where we could call the new API. Did you intentionally exclude them:
https://github.com/dotnet/runtime/search?l=C%23&q=.GetBits

For example:

runtime/src/libraries/System.Data.Odbc/src/System/Data/Odbc/OdbcParameterHelper.cs

Line 232 in 4f9ae42

return (byte)((decimal.GetBits((decimal)value)[3] & 0x00ff0000) >> 0x10);

runtime/src/libraries/System.Data.OleDb/src/DbParameterHelper.cs

Line 241 in 4f9ae42

return (byte)((decimal.GetBits((decimal)value)[3] & 0x00ff0000) >> 0x10);

runtime/src/libraries/System.Private.Xml/src/System/Xml/Xsl/IlGen/GenerateHelper.cs

Line 943 in 4f9ae42

int[] bits = decimal.GetBits(dec);

[stephentoub]

Did you intentionally exclude them

Yes, two of those target netstandard2.0. The other one is a code gen tool for which we have no tests and in which such a change won't be at all measurable.

[maryamariyan]

@ahsonkhan I haven't enabled it on the bot yet. (happens once every two days instead)

[maryamariyan]

@ahsonkhan I just enabled auto-labeling PRs with "new-api-needs-documentation" label. Should show up on future PRs.