Replace uses of boost::filesystem with fs

Step two in abstracting away boost::filesystem.

To repeat this, simply run:
```
git ls-files \*.cpp \*.h | xargs sed -i 's/boost::filesystem/fs/g'
```

src/test/testutil.h

@@ -10,6 +10,6 @@
 
 #include "fs.h"
 
-boost::filesystem::path GetTempPath();
+fs::path GetTempPath();
 
 #endif // BITCOIN_TEST_TESTUTIL_H