Keep operator= on FnRefs

The assignment is fine if it's from an already created FnRef that
is coming in from a higher stack frame. It's the construction of
a FnRef outside of a function parameter (ie. not on a stack frame
boundary to keep any temporary alive) that is the problem.

sus/fn/fn_ref.h

@@ -96,9 +96,10 @@ class [[sus_trivial_abi]] FnRef<R(CallArgs...)> {
     ::sus::check(invoke_);  // Catch use-after-move.
   }
   constexpr FnRef& operator=(FnRef&& o) noexcept {
-    // Assigning to an lvalue from a lambda would cause the lambda to be dropped
-    // but the FnRef would still hold a reference to it.
-    sus::unreachable();
+    storage_ = o.storage_;
+    invoke_ = ::sus::mem::replace(o.invoke_, nullptr);
+    ::sus::check(invoke_);  // Catch use-after-move.
+    return *this;
   }
 
   // Not copyable.
@@ -248,9 +249,10 @@ class [[sus_trivial_abi]] FnMutRef<R(CallArgs...)> {
     ::sus::check(invoke_);  // Catch use-after-move.
   }
   constexpr FnMutRef& operator=(FnMutRef&& o) noexcept {
-    // Assigning to an lvalue from a lambda would cause the lambda to be dropped
-    // but the FnMutRef would still hold a reference to it.
-    sus::unreachable();
+    storage_ = o.storage_;
+    invoke_ = ::sus::mem::replace(o.invoke_, nullptr);
+    ::sus::check(invoke_);  // Catch use-after-move.
+    return *this;
   }
 
   // Not copyable.
@@ -409,9 +411,10 @@ class [[sus_trivial_abi]] FnOnceRef<R(CallArgs...)> {
     ::sus::check(invoke_);  // Catch use-after-move.
   }
   constexpr FnOnceRef& operator=(FnOnceRef&& o) noexcept {
-    // Assigning to an lvalue from a lambda would cause the lambda to be dropped
-    // but the FnOnceRef would still hold a reference to it.
-    sus::unreachable();
+    storage_ = o.storage_;
+    invoke_ = ::sus::mem::replace(o.invoke_, nullptr);
+    ::sus::check(invoke_);  // Catch use-after-move.
+    return *this;
   }
 
   // Not copyable.

tools/run_subdoc.sh

@@ -11,4 +11,4 @@ out/subdoc/subdoc -p out --out docs \
     --project-logo logo.png \
     --project-name Subspace \
     --ignore-bad-code-links \
-   i32_unittest compat_range boxed choice_unit
+   i8_unittest