Use syntax sugar in VariantHashTable

table.get/set => table[]
table.contains => in table

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
VariantHashTable.has_key returns false for defaults

VariantHashTable.has_key has to return true only if there is a real
non-default value, but it returned true also for keys with only default
value. This has been fixed by tracking which keys have been set by
set_value() method.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>