Move Eigen::all,last,lastp1,lastN to Eigen::placeholders::.

These names are so common, IMO they should not exist directly in the
`Eigen::` namespace.  This prevents us from using the `last` or `all`
names for any parameters or local variables, otherwise spitting out
warnings about shadowing or hiding the global values.  Many external
projects (and our own examples) also heavily use
```
using namespace Eigen;
```
which means these conflict with external libraries as well, e.g.
`std::fill(first,last,value)`.

It seems originally these were placed in a separate namespace
`Eigen::placeholders`, which has since been deprecated.  I propose
to un-deprecate this, and restore the original locations.

These symbols are also imported into `Eigen::indexing`, which
additionally imports `fix` and `seq`. An alternative is to remove the
`placeholders` namespace and stick with `indexing`.

NOTE: this is an API-breaking change.

Fixes #2321.

test/stl_iterators.cpp

@@ -133,6 +133,7 @@ void test_stl_iterators(int rows=Rows, int cols=Cols)
   ColMatrixType A = ColMatrixType::Random(rows,cols);
   const ColMatrixType& cA(A);
   RowMatrixType B = RowMatrixType::Random(rows,cols);
+  using Eigen::placeholders::last;
   
   Index i, j;