Move ThunkBranch struct impl into Thunk::Branch

ThunkBranch standalone struct was not as useful as I was expecting it
would be. Its logic is now directly stored in Thunk::Branch variant

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile/list.rs

@@ -1,5 +1,5 @@
 use crate::{
-    compile::{Compile, thunk::{Thunk, ThunkBranch}},
+    compile::{Compile, thunk::Thunk},
     obj::{prelude::*, reserved::*},
     syn::{ast::*, visit::*},
     vm::inst::Inst,
@@ -89,11 +89,11 @@ impl List {
                 ]);
                 let thunk_true = body.thunkify(compile).into();
                 let thunk_false = el.thunkify(compile).into();
-                Thunk::Branch(ThunkBranch {
+                Thunk::Branch {
                     preamble: preamble.into(),
                     thunk_true,
                     thunk_false,
-                })
+                }
             }
             List::Lambda { params, expr } => {
                 // TODO(fun) : need captures for functions, built dynamically (or statically?)