Release v12.0.0

This release makes Nancy treat symlinks like UnionFS: when the target of a symlink in one input tree is overridden by a file in another input tree, that overriding is respected. Previously, the symlink would have been followed to its original target.

This change necessitated one backwards-incompatible change: the NANCY_INPUT environment variable now refers to the input file currently being processed, not the input root under which that file is found.