GZip File Reading

[dsimcha]

I noticed last night that Phobos actually has all the machinations 
required for reading gzipped files, buried in etc.c.zlib.  I've wanted a 
high-level D interface for reading and writing compressed files with an 
API similar to "normal" file I/O for a while.  I'm thinking about what 
the easiest/best design would be.  At a high level there are two designs:

1.  Hack std.stdio.file to support gzipped formats.  This would allow an 
identical interface for "normal" and compressed I/O.  It would also 
allow reuse of things like ByLine.  However, it would require major 
refactoring of File to decouple it from the C file I/O routines so that 
it could call either the C or GZip ones depending on how it's 
configured.  Probably, it would make sense to make an interface that 
wraps I/O functions and make an instance for C and one for gzip, with 
bzip2 and other goodies possibly being added later.

2.  Write something completely separate.  This would keep std.stdio.File 
doing one thing well (wrapping C file I/O) but would be more of a PITA 
for the user and possibly result in code duplication.

I'd like to get some comments on what an appropriate API design and 
implementation for writing gzipped files would be.  Two key requirements 
are that it must be as easy to use as std.stdio.File and it must be easy 
to extend to support other single-file compression formats like bz2.