Implement decimal benchmarks

[adamsitnik]

Today I was porting some of the Utf8Formatter benchmarks and I have realized that we have only one Decimal benchmark

https://github.com/dotnet/corefx/blob/79ef29bcc1b9e5f2d7ef595e136771c89eb1bce8/src/System.Runtime/tests/Performance/Perf.Decimal.cs

which I have ported to

performance/src/benchmarks/micro/corefx/System.Runtime/Perf.Double.cs

Line 46 in 5c02f92

public string Decimal_ToString() => decimalNum.ToString();

some time ago

We should definitely have more ;)

/cc @danmosemsft @tannergooding

[tannergooding]

CC. @pentp who has done much of the existing work on decimal.

[adamsitnik]

@stephentoub have implemented some benchmarks for decimal math operations in #478, I think that we can close this issue now.