update matrix API to allow both rows and columns

Author: alyst

inst/include/cpp11/matrix.hpp

@@ -140,12 +140,17 @@ class matrix {
 
   r_vector<r_string> names() const { return SEXP(vector_.names()); }
 
-  row operator[](const int pos) { return {*this, pos}; }
+  row row_at(const int pos) { return {*this, pos}; }
+  column column_at(const int pos) { return {*this, pos}; }
 
-  T operator()(int row, int col) { return vector_[row + (col * nrow_)]; }
+  T at(int row, int col) { return vector_[row + (col * nrow_)]; }
+  T operator()(int row, int col) { return at(row, col); }
 
-  row begin() { return {*this, 0}; }
-  row end() { return {*this, nrow_}; }
+  row rows_begin() { return {*this, 0}; }
+  row rows_end() { return {*this, nrow()}; }
+
+  column columns_begin() { return {*this, 0}; }
+  column columns_end() { return {*this, ncol()}; }
 };
 
 using doubles_matrix = matrix<r_vector<double>, double>;