Use a id_map implementation for SwitchMap

That way parameter definition order is respected when writing
command doc strings.

src/id_map.hh

@@ -18,6 +18,9 @@ public:
     using iterator = typename container_type::iterator;
     using const_iterator = typename container_type::const_iterator;
 
+    id_map() = default;
+    id_map(std::initializer_list<value_type> val) : m_content{val} {}
+
     void append(const value_type& value)
     {
         m_content.push_back(value);
@@ -91,6 +94,8 @@ public:
             prefix, cursor_pos, [](const value_type&) { return true; });
     }
 
+    bool empty() const { return m_content.empty(); }
+
     iterator       begin()       { return m_content.begin(); }
     iterator       end()         { return m_content.end(); }
     const_iterator begin() const { return m_content.begin(); }

src/parameters_parser.hh

@@ -2,11 +2,10 @@
 #define parameters_parser_hh_INCLUDED
 
 #include "exception.hh"
+#include "id_map.hh"
 #include "memoryview.hh"
 #include "string.hh"
 
-#include <unordered_map>
-
 namespace Kakoune
 {
 
@@ -40,7 +39,7 @@ struct SwitchDesc
     String description;
 };
 
-using SwitchMap = std::unordered_map<String, SwitchDesc>;
+using SwitchMap = id_map<SwitchDesc>;
 
 String generate_switches_doc(const SwitchMap& opts);