refactor: migrate ast lowering pass to ErrorGuaranteed (#81)

This is a first pass over the module. There are still some unnecessary
Result types being returned here, but that's cleanup for another day.

compiler/eight-driver/src/operations/ast_lower.rs

@@ -11,7 +11,7 @@ impl<'c> PipelinePass<'c, AstTranslationUnit<'c>, HirModule<'c>> for AstLowerPas
         input: AstTranslationUnit<'c>,
     ) -> Result<HirModule<'c>, PipelineError> {
         let mut lowering_pass = AstLoweringPass::new(pipeline.session);
-        let module = lowering_pass.visit_translation_unit(&input)?;
+        let module = lowering_pass.visit_translation_unit(input)?;
         Ok(module)
     }
 }

compiler/eight-middle/src/hir/builder.rs

@@ -16,7 +16,7 @@ use crate::hir::{
     HirReferenceExpr, HirReturnStmt, HirStmt, HirTrait, HirTraitSignature,
 };
 use crate::hir::{HirReferenceSymbol, HirTy};
-use crate::{HirResult, LinkageType};
+use crate::LinkageType;
 use eight_support::span::Span;
 use std::collections::BTreeMap;
 
@@ -33,10 +33,10 @@ impl<'hir> HirBuilder {
     ///
     /// This saves us from having to write `.iter().map(|x| ...).collect()` every time we want to
     /// collect something into a vector.
-    pub fn build_vec<A, B>(
+    pub fn build_vec<A, B, E>(
         iter: impl IntoIterator<Item = A>,
-        f: impl FnMut(A) -> HirResult<B>,
-    ) -> HirResult<Vec<B>> {
+        f: impl FnMut(A) -> Result<B, E>,
+    ) -> Result<Vec<B>, E> {
         iter.into_iter().map(f).collect()
     }