Prepare for 0.22.0 release (sicmutils#500)

prepare for 0.21.1 release (sicmutils#482)

Prepare for 0.21.0 release (sicmutils#475)

release 0.20.1 prep

release (sicmutils#395)

version bump

fix advanced compilation issues with Fraction (sicmutils#371)

0.19.0 release prep (sicmutils#369)

release (sicmutils#344)

prep (sicmutils#333)