Convert Decimal to all-managed implementation

[danmoseley]

CoreCLR's is in
https://github.com/dotnet/coreclr/blob/master/src/System.Private.CoreLib/src/System/Decimal.cs
https://github.com/dotnet/coreclr/blob/master/src/classlibnative/bcltype/decimal.cpp

The place to start is by grabbing the code verbatim from CoreRT and measuring performance, then optimizing to fix any regression:

https://github.com/dotnet/corert/blob/master/src/System.Private.CoreLib/src/System/Decimal.cs
https://github.com/dotnet/corert/blob/master/src/System.Private.CoreLib/src/System/Decimal.DecCalc.cs

Ensuring tests have good coverage, of course.

[danmoseley]

Once this is done the resulting implementation should move to System.Private.CoreLib/shared/System/Decimal.cs and both CoreCLR and CoreRT will use it.

[pentp]

dotnet/corert#4452 and dotnet/corert#4997 have most of the performance work done and the remaining performance regressions affect only x86 (everything else is already faster than the current native implementation). The following table has the remaining methods that need work, in particular, ADD and MUL are significantly slower.
(updated - FromDouble, HashCode and DIV removed)

32-bit x86

ADD	Mean	Error	StdDev	Scaled
Native	28.43 ns	0.0251 ns	0.0090 ns	1.00
CoreRT	64.05 ns	0.3440 ns	0.1227 ns	2.25
CoreRT2	35.95 ns	0.0438 ns	0.0156 ns	1.26

MUL	Mean	Error	StdDev	Scaled
Native	19.23 ns	0.0267 ns	0.0069 ns	1.00
CoreRT	47.70 ns	0.3100 ns	0.1106 ns	2.48
CoreRT2	28.09 ns	0.0383 ns	0.0137 ns	1.46

Negate	Mean	Error	StdDev	Scaled
Native	1.744 ns	0.0029 ns	0.0007 ns	1.00
CoreRT	7.583 ns	0.0181 ns	0.0065 ns	4.35
CoreRT2	1.855 ns	0.0012 ns	0.0003 ns	1.06

[pentp]

Test coverage should be pretty exhaustive already from dotnet/corefx#24053 and dotnet/corefx#26338.

[jkotas]

Also, compat issue with the CoreRT implementation needs to be addressed before this can land in CoreCLR: dotnet/corert#4994

[pentp]

I'll start working on making decimal a readonly struct in corert so that dotnet/corert#4994 can be resolved before decimal is shared with CoreCRL.