Prep 10.1.0 release

Bump version & update changelog

Update changelog for v9.0.0

Bump version number for 8.1.0 release

Change quadsegs from 0 to 8 in buffer(by:)

Relax Carthage dependency requirement

Increment version number for CocoaPods

Merge pull request GEOSwift#211 from GEOSwift/validity

Add isValid, isValidReason, and isValidDetail

Merge pull request GEOSwift#202 from alexander-prokopchuk/feature/add…

…-makeValid

Add makeValid

Merge pull request GEOSwift#201 from GEOSwift/geoswift-7

GEOSwift 7