add component 'output_buffer'

[hansu]

This adds a component that behaves like a tristate buffer, but the outputs will be zero instead of high-Z.

I haven't found a better name. Could also be named 'output_switch' or 'multiple_output_buffer' ...

                            ┌───────────┐
  output-buffer.N.in-0 ─────┤  ──[>]──  ├───── output-buffer.N.out-0
                            │   ┌─┘     │
  output-buffer.N.in-1 ─────┤  ─┼[>]──  ├───── output-buffer.N.out-1
                            │   ├─┘     │
  output-buffer.N.in-2 ─────┤  ─┼[>]──  ├───── output-buffer.N.out-2
                            │   ├─┘     │
                   ... ─────┤   :       ├───── ...
                            │   :       │
  output-buffer.N.in-M ─────┤  ─┼[>]──  ├───── output-buffer.N.out-M
                            │   ├─┘     │
                enable ─────┤  ─┘       │
                            └───────────┘