Use more efficient popcount

[szarnyasg]

Try __builtin_popcount for bitwise MSBFS

[marci543]

The unary op. should be built into GraphBLAS to avoid the overhead of function call.
Apply + built-in custom operator should be also considered instead of using materialized vector for scalar * vector operation.

[szarnyasg]

First, go with apply + UDF. This alone should give some a measurable speedup compared to the current naive popcount.

[szarnyasg]

The next SS:GrB will hopefully have a built-in popcount operator. Once that's out (#14), this will be trivial to resolve.