Fastback

This is a follow up bug patch release for 3.5.0.

Community

• Add sass.cr to implementations list (@straight-shoota, #2574)

Fixes

• Fix compiler warnings (@mgreter, #2575)
• Fix double free when run in concurrent processes (@mgreter, #2579)
• Fix units sometimes being dropped in math operations (@mgreter, #2582)
• Fix missing error for mixins defined within mixins (@mgreter, #2558)