Operations exists() and length() on Path

[StefanOltmann]

I want the operations exists() and length() from java.io.File on the common Path object to check the existence of a provided file before opening it an causing an error.

I don't know how to check existence in the native posix code, but a least for file size the fseek() & ftell() functions can be used.

[fzhinkin]

There's a PR extending file systems support, exists is already there. I'll add a way to get a file size there.