WebSocket refactoring and tests:

websocket:

* Move echo server to test/
* Fix warnings
* Fix maskgen being uncopyable
* Simplify utf8_checker special member declarations
* Fix stream move assignable when owning the next layer
* Add javadocs for stream special members
* Add stream unit tests
* Move throwing member definitions to the .ipp file
* Use get_lowest_layer in stream declaration
* Perform type checks at each call site instead of constructor
* Demote close_code to a non-class enum:
    Otherwise, application specific close codes
    cannot be assigned without using static_cast.

core:

* Add streambuf_readstream special members tests
* Add move assignment operator to streambuf_readstream
* Add detail/get_lowest_layer trait
* Add static_string tests
* Move static_string from websocket to core

include/beast/impl/streambuf_readstream.ipp

@@ -10,7 +10,6 @@
 
 #include <beast/bind_handler.hpp>
 #include <beast/handler_alloc.hpp>
-#include <beast/type_check.hpp>
 
 namespace beast {
 
@@ -158,8 +157,6 @@ streambuf_readstream<Stream, Streambuf>::
 streambuf_readstream(Args&&... args)
     : next_layer_(std::forward<Args>(args)...)
 {
-    static_assert(is_Stream<next_layer_type>::value,
-        "Stream requirements not met");
     static_assert(is_Streambuf<Streambuf>::value,
         "Streambuf requirements not met");
 }
@@ -173,6 +170,8 @@ async_write_some(ConstBufferSequence const& buffers,
         typename async_completion<
             WriteHandler, void(error_code)>::result_type
 {
+    static_assert(is_AsyncWriteStream<next_layer_type>::value,
+        "AsyncWriteStream requirements not met");
     static_assert(is_ConstBufferSequence<
         ConstBufferSequence>::value,
             "ConstBufferSequence requirements not met");
@@ -190,6 +189,8 @@ streambuf_readstream<Stream, Streambuf>::
 read_some(
     MutableBufferSequence const& buffers)
 {
+    static_assert(is_SyncReadStream<next_layer_type>::value,
+        "SyncReadStream requirements not met");
     static_assert(is_MutableBufferSequence<
         MutableBufferSequence>::value,
             "MutableBufferSequence requirements not met");
@@ -207,6 +208,8 @@ streambuf_readstream<Stream, Streambuf>::
 read_some(MutableBufferSequence const& buffers,
     error_code& ec)
 {
+    static_assert(is_SyncReadStream<next_layer_type>::value,
+        "SyncReadStream requirements not met");
     static_assert(is_MutableBufferSequence<
         MutableBufferSequence>::value,
             "MutableBufferSequence requirements not met");
@@ -239,6 +242,8 @@ async_read_some(
             typename async_completion<
                 ReadHandler, void(error_code)>::result_type
 {
+    static_assert(is_AsyncReadStream<next_layer_type>::value,
+        "Stream requirements not met");
     static_assert(is_MutableBufferSequence<
         MutableBufferSequence>::value,
             "MutableBufferSequence requirements not met");