add `enumerate`

Given an 'iterable', return a new iterable which iterates over a pair of index and elements of the original 'iterable'.

For example:

   for ( auto [i,s] : LHCb::range::enumerate( std::array{0,1,4,9,16,25} ) ) {
       assert( i*i == s );
   }

The index starts, by default, at 0, and has a type of size_t but this can be customised as well:

   for ( auto [i,s] : LHCb::range::enumerate( std::array{4u,9u,16u,25u,36u}, 2u ) ) {
       assert( i*i == s );
   }