Use function view when iterating over block symbols

Use function view when iterating over block symbols

This changes iterate_over_block_local_vars and
iterate_over_block_arg_vars to take a gdb::function_view rather than a
function pointer and a user-data.  In one spot, this allows us to
remove a helper structure and helper function.  In another spot, this
looked more complicated, so I changed the helper function to be an
"operator()" -- also a simplification, just not as big.