Master: Reimplement "Enable string import of single files" so that it doesn't break PIKE_MODULE_RELOC

Annotations are now resolved and get proper links.

This is needed by Stdio.FakePipe.

It is now possible to override an annotation with a @decl-directive.

Require Stdio.File to comply with the Stdio.NonblockingStream API.

PMOD_EXPORT does not imply extern...

Fixes linking issues on platforms that don't use FORTRAN-style
COMMON-segments (ie with eg gcc-10).