populate yas--exported-syms based on sym prefix

Rename what was formerly yas--exported-syms to yas--backported-syms,
and remove yas--dont-backport.
yas-text export twice

Something different between line 4521 and line 4528? I am not sure about this.
Merge pull request #446 from dongxiong/patch-2

Remove one extra `yas-text` exportation
Merge pull request #442 from npostavs/auto-export

populate yas--exported-syms based on sym prefix