This CL changes:
1. add function bitmap_to_string and bitmap_from_string, which will
convert a bitmap to/from string which contains all bit in bitmap
2. add function murmur_hash3_32, which will compute murmur hash for
input strings
3. make the function cast float to string the same with user result
logic
This CL make bitmap_count, bitmap_union, and bitmap_union_count accept any expression whose return type is bitmap as input so that we can support flexible bitmap expression such as bitmap_count(bitmap_and(to_bitmap(1), to_bitmap(2))).
This CL also create separate documentation for each bitmap UDF to conform with other functions.
Add a new type: Object. Currently, it's mainly for complex aggregate metrics(HLL , Bitmap).
The Object type has the following constraints：
1 Object type could not as key column type
2 Object type doesn't support all indices (BloomFilter, short key, zone map, invert index)
3 Object type doesn't support filter and group by
In the implementation：
The Object type reuse the StringValue and StringVal, because in storage engine, the Object type is binary, it has a pointer and length.
