LDC 1.28.0-beta1

[kinke]

Glad to announce the first beta for LDC 1.28 - some highlights:

* Based on D 2.098.0-beta.2+ (today's stable).
* Dynamic casts across binary boundaries (DLLs etc.) now work.
* Windows: `-dllimport=defaultLibsOnly` doesn't require 
`-linkonce-templates` anymore.

Full release log and downloads: 
https://github.com/ldc-developers/ldc/releases/tag/v1.28.0-beta1

Please help test, and thanks to all contributors & sponsors!