Port Enum performance parsing/formatting optimizations from CoreCLR

[jkotas]

Enum performance parsing/formatting optimizations done in CoreCLR:

dotnet/coreclr#9340
dotnet/coreclr#6645
dotnet/coreclr#3061
dotnet/coreclr#81

We should make sure to port these optimizations; and verify that the Enum parsing/formatting performance is on par with CoreCLR.

[MichalStrehovsky]

@jkotas Can we close this with 4b70bdb?

[jkotas]

Yes