Improve StreamWriter format perf (dotnet/coreclr#19235)
Improve StreamWriter format perf (dotnet/coreclr#19235)

* Improve StreamWriter format perf

Override the format overloads for TextWriter and skip the extra string allocation for the formatted string by using StringBuilder directly, copying straight to the output buffer.

Improves performance roughtly 10% and cuts allocations by 10x and up (formatting a string into a string goes to *zero* allocations).

* Fix copy/paste slipup- test added to CoreFX PR

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

ViktorHofer/corefx,mmitche/corefx,wtgodbe/corefx,ericstj/corefx,BrennanConroy/corefx,ericstj/corefx,mmitche/corefx,ViktorHofer/corefx,wtgodbe/corefx,wtgodbe/corefx,ptoonen/corefx,BrennanConroy/corefx,Jiayili1/corefx,ViktorHofer/corefx,wtgodbe/corefx,ptoonen/corefx,ptoonen/corefx,ptoonen/corefx,shimingsg/corefx,mmitche/corefx,Jiayili1/corefx,mmitche/corefx,ericstj/corefx,Jiayili1/corefx,mmitche/corefx,mmitche/corefx,ericstj/corefx,wtgodbe/corefx,BrennanConroy/corefx,ViktorHofer/corefx,shimingsg/corefx,shimingsg/corefx,mmitche/corefx,Jiayili1/corefx,ericstj/corefx,Jiayili1/corefx,shimingsg/corefx,ViktorHofer/corefx,shimingsg/corefx,ViktorHofer/corefx,Jiayili1/corefx,ptoonen/corefx,wtgodbe/corefx,ericstj/corefx,wtgodbe/corefx,ptoonen/corefx,shimingsg/corefx,ViktorHofer/corefx,Jiayili1/corefx,ericstj/corefx,ptoonen/corefx,shimingsg/corefx