Store string length as an int instead of size_t.
Store string length as an int instead of size_t.

Saves a few bits, and avoids some signed/unsigned conversions.
It's also consistent with other places string lengths are stored in the VM.

Rohansi/wren,munificent/wren,Nave-Neel/wren,munificent/wren,bigdimboom/wren,foresterre/wren,minirop/wren,Rohansi/wren,minirop/wren,bigdimboom/wren,foresterre/wren,Nelarius/wren,munificent/wren,lluchs/wren,minirop/wren,Nave-Neel/wren,Rohansi/wren,munificent/wren,lluchs/wren,Rohansi/wren,foresterre/wren,munificent/wren,Nave-Neel/wren,Nelarius/wren,Nelarius/wren,Nelarius/wren,foresterre/wren,munificent/wren,minirop/wren,Nave-Neel/wren,bigdimboom/wren,lluchs/wren,lluchs/wren,Nelarius/wren,foresterre/wren,minirop/wren,bigdimboom/wren