[IR] Explicit Stream Type in MLIR

[chhzh123]

Currently, Allo represents FIFO as an attributed memref (e.g., memref<16x16xf32, "stream:2">), but this actually does not capture the semantics of a FIFO and cannot support bundling data or structs in a stream to transfer. Therefore, we may need an explicit stream type in Allo to model this dataflow semantics, which can also simplify the codegen of these buffers.

[chhzh123]

This is fixed by #249.