Singular.jl 0.18.0 released

Today Singular.jl 0.18.0 has been released.

Diff since v0.17.0

Merged pull requests:

• add mstd, divrem (#625) (@hannes14)
• CI: enable depwarn: error for testsuite (#626) (@fingolfin)
• Remove unused AsEquivalent* conversion methods (#630) (@fingolfin)