In Elixir, you can't call a protocol on a module name - for instance
when calling an implemented protocol with a struct, you have to pass
the struct data as the first argument, not the struct module.

When parsing some markup with a custom type, Mecto doesn't have the
data, and can't just try create a struct from the module name as not
all custom types will be structs (also it's a bit hacky).

There are a few ways around this, but as this is a library a lot of
them aren't feasible (you can't create a behaviour in a module from
another project, for instance). The simplest solution seems to be
using `protocol_ex`.