LDC 1.20.0-beta1

[kinke]

Glad to announce the first beta for LDC 1.20:

* Based on D 2.090.0+ (recent DMD stable).
* Mac: support for embedding `pragma({lib,linkerDirective}, ...)` 
in object files, e.g., `pragma(linkerDirective, "-framework", 
"CoreFoundation");`.
* Linux: fix potential stack overflows in GC worker threads.
* CLI fix for options specified with 2 (not just 1) leading 
dashes, e.g., --lowmem.

Full release log and downloads: 
https://github.com/ldc-developers/ldc/releases/tag/v1.20.0-beta1

Please help test, and thanks to all contributors!