cope/avoid datetime objects; aarch64/arm64 wheels & containers

Add linux aarch64 wheel build support (#420) 5569e28 @odidev @mr-c
Build mypyc-enable Alpine linux docker container & publish it 07aab3a 2452fc2 9fa9171 @odidev @mr-c

Java codegen: cope with anonymous types a9f213c @mr-c

JSON dumping: cope with datetimes when dumping c1a1fc8 49d107b @mr-c
YAML loading: avoid datetime objects e2479c4 @mr-c