Vim2 improve buffer navigation

This pull requests improves navigation between buffers by supporting two new Ex commands and two new normal mode mappings.

The Ex commands are `:buffer` and `:buffers`. `:buffers` creates a new buffer to display a list of all the buffers. The status line is not used as it currently doesn't support newlines in the status. The output of `:buffers` includes the buffer number and the buffer name. The `:buffer` command switches to the specified buffer. The buffer can be specified either by the buffer number or by the buffer name.

Neither `:buffer` nor `:buffers` are fully compatible with Vim. The incompatibilities are spelled out in the commit messages.

I've also added two normal mode mappings: `<C-^>` and its synonym `<C-6>`. These both jump to the alternate (last used) buffer when no count is given or to the Nth file in the buffer access list when a count is given. The buffer access list is maintained on a per-window basis which is compatible with Vim.