StaticOnly should be enabled by default

[jvican]

Currently, if we want to ensure that picklers/unpicklers are generated statically at compile-time, this flag has to be imported in the scope.

I would propose to turn it around and create a flag called AllowDynamic that would make the program compile in case it has to generate picklers/unpicklers at runtime. Otherwise, StaticOnly would be chosen by default.

I think this is a reasonable way to deal with this, since most of our users expect static picklers.

[jvican]

WDYT @jsuereth ?

[andreaTP]

👍