make git_versions, normal_form and mantissa all optional
v2.020000

 [00 Major]
 - 'git_versions' is now both optional, and an ignored parameter.
 - 'normal_form' is now optional, and defaults to 'numify'
 - 'mantissa' is now optional, and defautls to 6.
 - Discrepancies with any of the above will be offloaded into the RecommendChanges plugin.

 [Dependencies::Stats]
 - Dependencies changed since 2.019000, see misc/*.deps* for details
 - develop: ↑1
 - runtime: ↑1
post-release cleanup