Hash-based .nib files

[lfkeitel]

Right now, .nib files are always checked to see if they match their source file. Only the modification time is checked. This may cause problems if a file is edited quickly, or a file is renamed but has the same timestamp. Hash-based nibs will detect even the smallest of changes, but at the expense of speed. Calculating even a sha1 hash takes time though it's probably negligible. This could be an optional mode of nib generation.

The current nib format should be updated to contain the size of a source file as well as the timestamp. The size would typically detect changes when files are moved around with the same timestamps.