Resolve generic arguments inside of nested fields

[SupernaviX]

Now correctly handles multi_asset<t> = { * bytes => { * bytes => t } }

Partially fixes #99

[rooooooooob]

Thanks for the contribution! I changed using_extern_generics to:

generic_map<t> = { * bytes => { * bytes => t } }

using_generics = [
  foo: extern_generic<external_foo>,
  map_uint: generic_map<uint>,
  ;map_text: generic_map<text>,
  ;map_foo: generic_map<foo>,
]

inside of tests/core/input.cddl to test it out. It looks like there are still some issues when building the resulting code. I'll take a closer look in the morning, but it probably has to do with how we handle serialization of maps.