[Optimizer] Low hanging fruit: Limit->Limit

[xcharleslin]

We don't currently fuse limits:

>>> daft.from_pydict({"a": [1, 2, 3, 4, 5]}).limit(4).limit(3).explain(show_optimized=True)
┌─GlobalLimit
│    output=[col(a)]
│    num=3
│    partitioning={'by': None, 'num_partitions': 1, 'scheme': PartitionScheme.Unknown}
│
├──LocalLimit
│    output=[col(a)]
│    num=3
│    partitioning={'by': None, 'num_partitions': 1, 'scheme': PartitionScheme.Unknown}
│
├──GlobalLimit
│    output=[col(a)]
│    num=4
│    partitioning={'by': None, 'num_partitions': 1, 'scheme': PartitionScheme.Unknown}
│
├──LocalLimit
│    output=[col(a)]
│    num=4
│    partitioning={'by': None, 'num_partitions': 1, 'scheme': PartitionScheme.Unknown}
│
└──InMemoryScan
     output=[col(a)]
     cache_id='2218181262134c9f99746c2052d611bf'
     partitioning={'by': None, 'num_partitions': 1, 'scheme': PartitionScheme.Unknown}

[samster25]

fixed by: #1730