Singular.jl 0.5.1 released

Today Singular.jl 0.5.1 has been released.

Diff since v0.5.0

Closed issues:

• Initializing a n_transExt with an fmpz broken in Julia 1.3 (#395)

Merged pull requests:

• functionality depending on next libsingular-julia version (#385) (@tthsqe12)
• add some needed gc protection (#399) (@tthsqe12)
• Add downstream (Oscar) testing (#401) (@thofma)
• saturation: also return number of iterations (#402) (@hannes14)
• Import and export constant_coefficient, add test. (#404) (@wbhart)
• CI: test with Julia 1.6 (#406) (@fingolfin)