v2.8.1: TypeScript Improvements

• When route parameters can be inferred from the path, they now have a type of { readonly [k in string]: string | undefined }. This takes into account optional segments that can be undefined #262, thanks @HansBrende
• Proper typings of Route children: it does not allow mixing render props and other elements together #263, thanks @HansBrende
• Switch now has less strict type of its children to allow conditional rendering and avoid confusion 8f943ab