Optimize for fast-path in String >> at: Avoid jumps in String >> at: for common case the receiver is an instance of String and thus has no instance variables. Similar as fix 877a8f1b326d for Arrays.

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@REM make using lcc compiler
@REM type lccmake, and wait...
@REM do not edit - automatically generated from ProjectDefinition
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make.exe -N -f bc.mak -DUSELCC=1 %*