[C++] Split flatbuffers.h into separate files (#6868)

* split flatbuffers.h into separate files

* wrong variable in cmakelists for android

* readded two accidentally deleted includes

* created buffer.h and moved buffer related things over

include/flatbuffers/registry.h

@@ -17,6 +17,7 @@
 #ifndef FLATBUFFERS_REGISTRY_H_
 #define FLATBUFFERS_REGISTRY_H_
 
+#include "flatbuffers/base.h"
 #include "flatbuffers/idl.h"
 
 namespace flatbuffers {
@@ -40,13 +41,13 @@ class Registry {
   bool FlatBufferToText(const uint8_t *flatbuf, size_t len, std::string *dest) {
     // Get the identifier out of the buffer.
     // If the buffer is truncated, exit.
-    if (len < sizeof(uoffset_t) + FlatBufferBuilder::kFileIdentifierLength) {
+    if (len < sizeof(uoffset_t) + kFileIdentifierLength) {
       lasterror_ = "buffer truncated";
       return false;
     }
     std::string ident(
         reinterpret_cast<const char *>(flatbuf) + sizeof(uoffset_t),
-        FlatBufferBuilder::kFileIdentifierLength);
+        kFileIdentifierLength);
     // Load and parse the schema.
     Parser parser;
     if (!LoadSchema(ident, &parser)) return false;