make Julia global RNG be an instance of MersenneTwister

The state of the global RNG was previously handled by libdSFMT.
This commit allows the global RNG to benefit speed improvements resulting
from the use of fill_array_* functions (cf. previous commit).

However, individual calls to rand() (i.e. producing one value instead of
filling an array) using the global RNG are now slower, as well as those filling
non-Float64 arrays.