fix(codec): Fix flat encoding and decoding of arbitrarily size integers (#378)

This commits fixes the flat encoding and decoding (and consequently,
  the zigzag) for large integers in the following ways:

  - It removes support for encoding and decoding i128 values.

  - It optionally (feature = "num-bigint") introduces encoding and
    decoding of large sized integers through the num-bigint::BigInt
    type.

  Without the feature enabled, it is still possible to encode and decode
  isize values; but the use of i128 is now prohibited (as it would
  overflow on boundaries) in favor of arbitrarily sized integers.

  The commit also introduces a missing property roundtrip for encoding
  and decoding large integers, which was missing and thus, failed to
  identify the overflow problem.

  See related issue: https://github.com/aiken-lang/aiken/issues/796

pallas-codec/Cargo.toml

@@ -14,9 +14,13 @@ authors = [
   "Kasey White <kwhitemsg@gmail.com>",
 ]
 
+[features]
+default = []
+
 [dependencies]
 hex = "0.4.3"
 minicbor = { version = "0.20", features = ["std", "half", "derive"] }
+num-bigint = { version = "0.4.4", optional = true }
 serde = { version = "1.0.143", features = ["derive"] }
 thiserror = "1.0.39"