Rearrange some plugins to have important ones at top
Add vim-sexp and bisectly plugins

actually adding two plugins for vim-sexp
Make <Space> a no-op in visual and operator modes too
Fix select-mode mappings

By changing v*map to x*map in all cases where the mapping is to a printable character, and by adding a sunmap after each (nore)map.

I also moved some mappings that didn’t use `nore` into the section for that.
Upgrade bufexplorer to newer GitHub version
Remove the BufExplorer mapping

because the prefix isn’t unique, so it doesn’t take effect until I press another key that is not part of another mapping

I will use the plugin’s provided <Leader>bs and <Leader>bv  mappings instead
Add TODO about tracking down plugin bugs using `:smap`