Modest refactor of CapacityByteArrayOutputStream

Optimization for the CapacityByteArrayOutputStream class:

1. The functionality of currentSlabIndex is the same as currentSlab.position(), so there is no need to maintain the currentSlabIndex variable.
2. When writing an array of length equal to the remaining capacity of the buffer, there is no need to expand to a new buffer.
3. If the addSlab operation has already implemented safeguards using Math.addExact to prevent overflow of bytesAllocated and bytesUsed, it is unnecessary to perform additional checks during the write operation.