blas/gonum: add Ztrmm and Ztrsm

Created by: vladimir-ch

Closes #185 (closed) (sound of fanfares in the distance)

Also sneaked in some cleanup changes in Dtrmm and Dtrsm, including fixes to matrix strides in Dtrsm and Dsyr2k. Unfortunately the tests would require non-small changes to fail on this.

Please take a look.