fix const correctness in template_t

Without locking we can safely share only const objects between threads.
crow::mustache::template_t will typically be loaded once and then
reused many times across threads. Unfortunately the render() method
was not marked as const, even though it is const is practice. This commit
adds the missing const annotations to the methods involved.

Author: neumannt

include/crow/mustache.h

@@ -64,11 +64,11 @@ namespace crow
             }
 
         private:
-            std::string tag_name(const Action& action)
+            std::string tag_name(const Action& action) const
             {
                 return body_.substr(action.start, action.end - action.start);
             }
-            auto find_context(const std::string& name, const std::vector<context*>& stack, bool shouldUseOnlyFirstStackValue = false) -> std::pair<bool, context&>
+            auto find_context(const std::string& name, const std::vector<context*>& stack, bool shouldUseOnlyFirstStackValue = false) const -> std::pair<bool, context&>
             {
                 if (name == ".")
                 {
@@ -133,7 +133,7 @@ namespace crow
                 return {false, empty_str};
             }
 
-            void escape(const std::string& in, std::string& out)
+            void escape(const std::string& in, std::string& out) const
             {
                 out.reserve(out.size() + in.size());
                 for (auto it = in.begin(); it != in.end(); ++it)
@@ -153,7 +153,7 @@ namespace crow
                 }
             }
 
-            bool isTagInsideObjectBlock(const int& current, const std::vector<context*>& stack)
+            bool isTagInsideObjectBlock(const int& current, const std::vector<context*>& stack) const
             {
                 int openedBlock = 0;
                 int totalBlocksBefore = 0;
@@ -179,7 +179,7 @@ namespace crow
                 return false;
             }
 
-            void render_internal(int actionBegin, int actionEnd, std::vector<context*>& stack, std::string& out, int indent)
+            void render_internal(int actionBegin, int actionEnd, std::vector<context*>& stack, std::string& out, int indent) const
             {
                 int current = actionBegin;
 
@@ -323,7 +323,7 @@ namespace crow
                 auto& fragment = fragments_[actionEnd];
                 render_fragment(fragment, indent, out);
             }
-            void render_fragment(const std::pair<int, int> fragment, int indent, std::string& out)
+            void render_fragment(const std::pair<int, int> fragment, int indent, std::string& out) const
             {
                 if (indent)
                 {
@@ -339,7 +339,7 @@ namespace crow
             }
 
         public:
-            std::string render()
+            std::string render() const
             {
                 context empty_ctx;
                 std::vector<context*> stack;
@@ -349,7 +349,7 @@ namespace crow
                 render_internal(0, fragments_.size() - 1, stack, ret, 0);
                 return ret;
             }
-            std::string render(context& ctx)
+            std::string render(context& ctx) const
             {
                 std::vector<context*> stack;
                 stack.emplace_back(&ctx);