Fixed file modes and path separator issues for Win32

For every slurp, spew, and open, I modified them to use the 'raw' binmode.

For the Directory asset, I updated multiple places where it was mixing
filenames and URI paths, which is fine on unix but on windows requires
a forward/backward slash conversion.

Also updated each testcase to remove the workdir before running the test
so that results of a previous run don't interfere with subsequent tests.
Added the "." filter on paths to other places in the code where I join filenames

I needed a " grep { $_ ne '.' } " to prevent "./" prefixes in the built asset.
I'm not entirely sure if I need them elsewhere also, but I suspect so and it
doesn't break the testcases, so I added them.