Provide better errors on unknown type in cyclic definitions.

Let's consider the following case:

  ```
  type Var =
    Integer

  type Vars =
    List<Var>
  ```

  This incorrectly reports an infinite cycle; due to the inability to
  properly type-check `Var` which is also a dependent var of `Vars`. Yet
  the real issue here being that `Integer` is an unknown type.

  This commit also upgrades miette to 7.2.0, so that we can also display
  a better error output when the problem is actually a cycle.

crates/aiken-project/src/error.rs

@@ -61,7 +61,7 @@ pub enum Error {
     TomlLoading {
         path: PathBuf,
         src: String,
-        named: Box<NamedSource<String>>,
+        named: NamedSource<String>,
         location: Option<Span>,
         help: String,
     },
@@ -76,7 +76,7 @@ pub enum Error {
     Parse {
         path: PathBuf,
         src: String,
-        named: Box<NamedSource<String>>,
+        named: NamedSource<String>,
         #[source]
         error: Box<ParseError>,
     },
@@ -454,7 +454,7 @@ impl Diagnostic for Error {
             Error::Type { named, .. } => Some(named),
             Error::StandardIo(_) => None,
             Error::MissingManifest { .. } => None,
-            Error::TomlLoading { named, .. } => Some(named.as_ref()),
+            Error::TomlLoading { named, .. } => Some(named),
             Error::Format { .. } => None,
             Error::TestFailure { .. } => None,
             Error::Http(_) => None,