Modify DESCRIPTION

Depends: xts, Rcpp
linkingTo: xts, Rcpp

Create files in src directory

c xts_api.c #include <xts.h> #include <xts_stubs.c>

```cpp xts_api.h extern “C” { #define class xts_class #include <xts.h> #undef class }

inline SEXP install(const char* x) { return Rf_install(x); }

inline SEXP getAttrib(SEXP a, SEXP b) { return Rf_getAttrib(a, b); }

inline SEXP setAttrib(SEXP a, SEXP b, SEXP c) { return Rf_setAttrib(a, b, c); }

Without the macro, there will be  compile time error:

error: expected identifier before ‘)’ token

because `xts.h` use the keyword `class`.

Without the inline functions, there will be some compile time errors:

error: ‘install’ was not declared in this scope error: ‘getAttrib’ was not declared in this scope

Now, almost all API could be invoked in c++:

```cpp rcpp_test.cpp
#include <Rcpp.h>

#include "xts_api.h"

using namespace Rcpp;

RcppExport SEXP get_xts_index(SEXP x) {
  BEGIN_RCPP
  
  return GET_xtsIndex(x);
  
  END_RCPP
}

except SET_xtsIndexClass(x, value):

sh compile time error error: ‘xts_IndexvalueSymbol’ was not declared in this scope

I guess that we should replace xts_IndexvalueSymbol with xts_IndexClassSymbol