`AbsolutePath.Exists` should return `true` if the entity is a directory

[ForNeVeR]

I've had this simple program:

open TruePath
open TruePath.SystemIo

let repoRoot = AbsolutePath __SOURCE_DIRECTORY__ / ".."

let folders = [|
        repoRoot / "TdLib.Api" / "Objects"
        repoRoot / "TdLib.Api" / "Functions"
    |]
    for folder in folders do
        if folder.Exists() then
            printfn $"Removing folder \"{folder.Value}\"…"
            folder.DeleteDirectoryRecursively()
        else printfn $"Nothing to clean in \"{folder.Value}\": folder doesn't exist."

And it broke because folder.Exists() doesn't check for directory existence, but only for files.

Let's have separate explicit methods ExistsFile() and ExistsDirectory() (in fact we already have the latter), and make Exists() to check the entity for existence without checking for its exact kind.

[cnaples79]

I'm working on this.

[ForNeVeR]

I think it's already handled by #172.

(Sorry I am a bit overwhelmed by work at the moment, so didn't review that yet, but I will soon!)

[cnaples79]

@ForNeVeR Ah, I didn't see your comment in time..I opened the PR already. Sorry about that.