Closes joaotavora#36: use `scan-sexps` again in `autopair--up-list`

Closes joaotavora#35: redesign to make compatible with (slightly) old…

…er emacsen again

Also:
- remove unneeded hack for `global-hl-line-mode` compatibility
- fix indentation and other cosmetic fixes
- internal symbols are now prefixed "autopair--"
- remove byte-compilation warnings about cl-functions

Closes joaotavora#31: imitate isearch-mode-map when binding return an…

…d backspace.

- Also bump version to 0.5