abi: explicitly export intended symbols and hide others

This adds all exported symbols in the map files explicitly under
the following rules:
 - Symbols mentioned in internal headers go in a section which is
   valid only for testing, and linking with these symbols will break
   in library updates.
 - Symbols mentioned in installed headers go in the exported sections
   and are considered part of the ABI.
 - All internal symbols move to internal headers.
 - The _nettle_md5_compress and _nettle_sha1_compress become exported
   without the _nettle prefix, due to existing usage.