Add m.ignored_user_list support

Allows us to implement extra functionality and shortens the symbol
names. Does not work with gcc10.

Refactor special effect event handling

If an Unknown message is being constructed for sending, the content
may be empty, since the msgtype and body are exposed as C++ properties.
Previously, the to_json function assumed that the content could be
parsed as valid json, when in fact an empty content would throw an error.

Add support for unknown messages

We do retain some template instantiations by default, but only for types
actually used in the public API. If anyone wants to use the other ones,
they will need to import the "*_impl.hpp" defining their respective from
and to json functions. Everyone else should just try to parse only the
types we use in our variants and such.

fixes #93
fixes #80