Refactor AssignmentKind to allow backpassing on both let and expect.

The 3rd kind of assignment kind (Bind) is gone and now reflected through a boolean parameter. Note that this parameter is completely erased by the type-checker so that the rest of the pipeline (i.e. code-generation) doesn't have to make any assumption. They simply can't see a backpassing let or expect.

crates/aiken-lang/src/gen_uplc/builder.rs

@@ -4,8 +4,8 @@ use super::{
 };
 use crate::{
     ast::{
-        AssignmentKind, BinOp, ClauseGuard, Constant, DataTypeKey, FunctionAccessKey, Pattern,
-        Span, TraceLevel, TypedArg, TypedClause, TypedClauseGuard, TypedDataType, TypedPattern,
+        BinOp, ClauseGuard, Constant, DataTypeKey, FunctionAccessKey, Pattern, Span, TraceLevel,
+        TypedArg, TypedAssignmentKind, TypedClause, TypedClauseGuard, TypedDataType, TypedPattern,
         UnOp,
     },
     builtins::{bool, data, function, int, list, void},
@@ -68,7 +68,7 @@ pub enum HoistableFunction {
 #[derive(Clone, Debug)]
 pub struct AssignmentProperties {
     pub value_type: Rc<Type>,
-    pub kind: AssignmentKind,
+    pub kind: TypedAssignmentKind,
     pub remove_unused: bool,
     pub full_check: bool,
     pub msg_func: Option<AirMsg>,