Add utilities for working with signs

[dbarnett]

Vim's signs are useful but managed globally by default. It's hard to do things like clear all signs created by your plugin.

maktaba could have some utilities for working with signs.

[dbarnett]

Looks like vim added some built-in functions for working with signs in 8.1.0614 (see vim/vim#3652 and :h sign-functions-details), so this would be a polyfill around those and follow conventions from those signatures.