extend Memory backend to dealing with various needs

[KotorinMinami]

We know that qtrvsim uses a memorytree with a fixed depth of 6, which causes its memory model to only accept 32-bit memory access instructions, thus restricting 64-bit access. This limitation presents an issue in terms of educational rigor. Therefore, this PR makes corresponding adjustments so that the memory class adapts its memory access bit width according to xlen, while also addressing the #146