Tidy up tests and docs:

* Add handler_ptr test and increase coverage
* Add test for prepare_buffer
* Move is_call_possible tests to a .cpp file
* Tidy up docs and declarations

test/core/handler_alloc.cpp

@@ -6,23 +6,44 @@
 //
 
 // Test that header file is self-contained.
-#include <beast/core/to_string.hpp>
+#include <beast/core/handler_alloc.hpp>
 
 #include <beast/unit_test/suite.hpp>
-#include <boost/asio/buffer.hpp>
+#include <vector>
 
 namespace beast {
 
-class to_string_test : public beast::unit_test::suite
+class handler_alloc_test : public beast::unit_test::suite
 {
 public:
-    void run()
+    struct handler
     {
-        BEAST_EXPECT(to_string(boost::asio::const_buffers_1("x", 1)) == "x");
+        void
+        operator()() const
+        {
+        }
+    };
+
+    void
+    run() override
+    {
+        handler h;
+        handler h2;
+        handler_alloc<char, handler> a1{h};
+        handler_alloc<char, handler> a2{h2};
+        BEAST_EXPECT(a2 == a1);
+        auto a3 = a1;
+        BEAST_EXPECT(a3 == a1);
+        {
+            std::vector<char,
+                handler_alloc<char, handler>> v(a1);
+            v.reserve(32);
+            v.resize(10);
+        }
     }
 };
 
-BEAST_DEFINE_TESTSUITE(to_string,core,beast);
+BEAST_DEFINE_TESTSUITE(handler_alloc,core,beast);
 
 } // beast