Add acquire() to maybe_t

Easy way to pull the value out.

src/fish_tests.cpp

@@ -4491,6 +4491,12 @@ void test_maybe() {
     do_test(m4 && *m4 == "hi");
     maybe_t<std::string> m5 = m0;
     do_test(!m5);
+
+    maybe_t<std::string> acquire_test("def");
+    do_test(acquire_test);
+    std::string res = acquire_test.acquire();
+    do_test(!acquire_test);
+    do_test(res == "def");
 }
 
 void test_layout_cache() {

src/maybe.h

@@ -64,6 +64,14 @@ class maybe_t {
         return *reinterpret_cast<const T *>(storage);
     }
 
+    // Transfer the value to the caller.
+    T acquire() {
+        assert(filled && "maybe_t does not have a value");
+        T res = std::move(value());
+        reset();
+        return res;
+    }
+
     // Clear the value.
     void reset() {
         if (filled) {