Underlying CoreCLR support for new string slicing overloads (#15811)
Underlying CoreCLR support for new string slicing overloads (#15811)

* Underlying CoreCLR support for new string slicing overloads

https://github.com/dotnet/corefx/issues/25254

These add the underlying support for the fast versions of
these extension methods.

* Underlying CoreCLR support for new string slicing overloads

https://github.com/dotnet/corefx/issues/24072

These add the underlying support for the fast versions of
these extension methods.

Signed-off-by: dotnet-bot-corefx-mirror <03c4cb6107ae99956a7681f2c97ff7104809a2bd@microsoft.com>

Jiayili1/corefx,Jiayili1/corefx,Jiayili1/corefx,ViktorHofer/corefx,ericstj/corefx,ptoonen/corefx,ravimeda/corefx,ptoonen/corefx,ViktorHofer/corefx,BrennanConroy/corefx,Jiayili1/corefx,ptoonen/corefx,wtgodbe/corefx,ravimeda/corefx,zhenlan/corefx,ptoonen/corefx,ViktorHofer/corefx,wtgodbe/corefx,shimingsg/corefx,mmitche/corefx,mmitche/corefx,ravimeda/corefx,Jiayili1/corefx,ericstj/corefx,wtgodbe/corefx,ravimeda/corefx,ericstj/corefx,mmitche/corefx,BrennanConroy/corefx,Ermiar/corefx,Ermiar/corefx,ericstj/corefx,mmitche/corefx,Ermiar/corefx,shimingsg/corefx,ericstj/corefx,ptoonen/corefx,zhenlan/corefx,ericstj/corefx,wtgodbe/corefx,Ermiar/corefx,ViktorHofer/corefx,Jiayili1/corefx,shimingsg/corefx,shimingsg/corefx,zhenlan/corefx,ravimeda/corefx,Ermiar/corefx,shimingsg/corefx,wtgodbe/corefx,zhenlan/corefx,mmitche/corefx,zhenlan/corefx,ViktorHofer/corefx,ptoonen/corefx,ViktorHofer/corefx,zhenlan/corefx,wtgodbe/corefx,Ermiar/corefx,ravimeda/corefx,mmitche/corefx,shimingsg/corefx,ericstj/corefx,mmitche/corefx,zhenlan/corefx,ptoonen/corefx,Ermiar/corefx,wtgodbe/corefx,ViktorHofer/corefx,Jiayili1/corefx,shimingsg/corefx,BrennanConroy/corefx,ravimeda/corefx