Rework automatic serialization

[arnetheduck]

#86 introduced some support for disabling automatic serialization - however:

• using a macro cache has undesireable complexity of usage - for example, if a.nim calls setAutoSerialize(true) and b.nim doesn't, serialization in b.nim will work or not depending on import order and / or whether a was imported at all - for example, compiling a and b together will work, but compiling b on its own will not (this can be seen in the json-rpc test suite where test_router_rpc compiles when part of all_tests but not on its own because the automatic serialization is configured in some other test
• if two modules set opposite values, the semantics are unclear - having setAutoSerialize take a boolean parameter is difficult to understand and work with
• the API is unclear - why is setAutoSerialize different from useDefaultSerializationIn and why can't the latter be used for auto-serialization of primitives? the inconsistency makes the API hard to understand when there are two similar ways of doing the same thing (enabling "default" serialization for a type)