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
1 Because we don't support array type currently, so I use variable arguments instead.
2 intersect_count directly return final count, not bitmap like bitmap_union, because intersect_count return bitmap is more complex and need more serialize. If we really need bitmap format from intersect_count, we could do that in another PR and which won't have compatibility problems.
