Export functions for extracting exported declarations

Move functions for extracting exported declarations out of psc-docs and
into Language.PureScript so that they may be used in other programs,
such as Pursuit.

Refs purescript/pursuit#1
only use exported declarations in psc-docs

Previously was using the full list of declarations for some things,
which meant that (for example) a module which contained some
non-exported type declarations could end up with a "Types" header but
nothing underneath it.
fix a compile error
Merge pull request #778 from hdgarrood/export-helpers

Export functions for extracting exported declarations