WT construction

[simongog]

The construct method for WTs first loads the original data and then serializes it as an int_vector. The int_vector is not necessarily bit-compressed (e.g. elements of the int_vector might use more bits than necessary to represent the maximum value of the vector) and therefore could be the space bottleneck of the WT construction. This can be fixed by first calculating the maximum value in the vector and bit-compressed the input.