Bindings::get(): Add convenience method

This allows writing attribute lookups as

    if (auto name = value.attrs->get(state.sName))
      ...

src/libexpr/attr-set.hh

@@ -5,6 +5,7 @@
 #include "gc.hh"
 
 #include <algorithm>
+#include <optional>
 
 namespace nix {
 
@@ -68,6 +69,14 @@ public:
         return end();
     }
 
+    std::optional<Attr *> get(const Symbol & name)
+    {
+        Attr key(name, 0);
+        iterator i = std::lower_bound(begin(), end(), key);
+        if (i != end() && i->name == name) return &*i;
+        return {};
+    }
+
     iterator begin() { return &attrs[0]; }
     iterator end() { return &attrs[size_]; }