Cardano Updates

* @custom_serialize / @custom_deserialize DSL

e.g. `; @custom_serialize write_hex_bytes`

For specifying externally-provided functions for arbitrary
encodings/cbor details for (de)serialization.

Allowed at both the type-level (affecting everywhere it's used) or at
the field-level (overrides type-level if present).

Example use-case: CML's PlutusData's Bytes (and BigInt)  variant doesn't use arbitrary
CBOR bytes strings but instead follows a specific chunking format. We
used to hand code this but now we can just put this in the DSL.

This is particularily useful for people generating plutus-datum-based
CDDLs. It could also be used to allow for utf8 text (rust API) to be
(de)serialized to bytes to be encodable as a datum.

TODO:
[ ] tests for preserve-encodings
[ ] tests for tagged/otherwise extra encoding details over top of this

* preserve-encodings=true tests

* docs + more test cases + misc fixes

* clarify where the custom_serialize string comes from

* Better error checks/messages for file missing related errors to help
  debugging as the error was incredibly opaque as to where it
originated.

* Imports correctly from the common import override in `serialization.rs`
files instead of relying on the `import super::*;` which only works when
it's not overrided.