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Sasa v0.16.0 Released

Mainly a bugfix release, with only a few new features. As always, the docs are available here online, or as a compiled CHM file on sourceforge. Here's the changelog:

 * a few bugfixes to MIME parsing for header and word decoding (Thanks Evan!)
 * added combined SubstringSplit function for more space efficient parsing
 * explicitly parse e-mail addresses for more flexible address separators
 * NonNull now throws an exception if encapsulated value is null, which
   is used in the case when the instance is invalidly constructed (Thanks Mauricio!)
 * build now checks for the presence of the signing key and ignores it if
   it's not present
 * a more efficient Enum.HasFlag using codegen
 * added a new ImmutableAttribute which Sasa's runtime analyses respects
 * FilePath's encapsulated string now exposed as a property

Thanks to Evan/iaiken for a number of bug reports, and to Mauricio for feedback on the NonNull<T> pattern.

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