feat: Move flat en/de from aiken to pallas (#303)

Nothing new is going on within the code itself.
I simply popped the crate into pallas_codec
as a submodule `pallas_codec::flat`. I also moved
over the tests that we had in the crate. In general
this is in solid shape and hasn't had any changes for
months. That said there could be some things that require love
like dealing with BigInt.

Co-authored-by: Kasey White <kwhitemsg@gmail.com>

pallas-codec/src/lib.rs

@@ -1,3 +1,6 @@
+/// Flat encoding/decoding for Plutus Core
+pub mod flat;
+
 /// Shared re-export of minicbor lib across all Pallas
 pub use minicbor;