Implement set values by boolean indexing

`setitem` is now only support slice or int, we can implement set values by boolean indexing.

``` Python
In [1]: a = mt.random.rand(5, 5, chunk_size=3)

In [2]: a[a > 0.5] = 1

In [3]: a.execute()
Out[3]:
array([[1. , 0.4249057 , 1. , 1. , 1. ],
[1. , 0.13329489, 0.22285302, 0.16091528, 1. ],
[0.45884747, 0.20933834, 0.32894797, 0.37087946, 1. ],
[0.19214502, 1. , 1. , 0.28669773, 0.16544558],
[1. , 1. , 1. , 0.25927057, 1. ]])
```

Start by locating the existing setitem implementation and the indexing tests or entry points that currently handle slice and integer assignment. Reproduce the boolean-indexing example from the issue, then extend coverage so that assigning 1 to values selected by a boolean condition executes successfully and produces the expected array.

Written by the indexing model from the issue text.