Map wrappers: forward more of the std::map interface

This remains incomplete, forwarding as per users' needs rather than
forwarding the entire std::map interface, hopefully splitting the
difference between providing useful functionality and preserving the
possibility to switch out different types without undue difficulty.

Author: smowton

src/pointer-analysis/value_set.h

@@ -83,7 +83,12 @@ class value_sett
     size_t size() const { return data.size(); }
     bool empty() const { return data.empty(); }
 
+    void erase(unsigned i) { data.erase(i); }
+    void erase(iterator it) { data.erase(it); }
+
     objectt &operator[](unsigned i) { return data[i]; }
+    objectt &at(unsigned i) { return data.at(i); }
+    const objectt &at(unsigned i) const { return data.at(i); }
 
     template <typename It>
     void insert(It b, It e) { data.insert(b, e); }

src/util/numbering.h

@@ -152,6 +152,9 @@ class hash_numbering final
   T &operator[](size_type t) { return data[t]; }
   const T &operator[](size_type t) const { return data[t]; }
 
+  T &at(size_type t) { return data.at(t); }
+  const T &at(size_type t) const { return data.at(t); }
+
   size_type size() const { return data.size(); }
 
   iterator begin() { return data.begin(); }