Add fs.ResolvePath to resolve symbolic links (#275)

* Add `fs.ResolvePath` to resolve symbolic links

`filepath.EvalSymlinks` does not work well on Windows, and can enter
infinite loops in certain situations and error out.
Use Win32 API GetFinalPathNameByHandle to handle path resolution.

Implementation based off on: https://github.com/containerd/containerd/pull/5411

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

* PR: types, documentation

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

* remove unneded constant groups

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

* Attempt normalized path first

Update logic to try querying for normalized path initially, then use
opened path if access is denied.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

---------

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

internal/stringbuffer/wstring_test.go

@@ -0,0 +1,45 @@
+//go:build windows
+
+package stringbuffer
+
+import "testing"
+
+func Test_BufferCapacity(t *testing.T) {
+	b := NewWString()
+
+	c := b.Cap()
+	if c < MinWStringCap {
+		t.Fatalf("expected capacity >= %d, got %d", MinWStringCap, c)
+	}
+
+	if l := len(b.b); l != int(c) {
+		t.Fatalf("buffer length (%d) and capacity (%d) mismatch", l, c)
+	}
+
+	n := uint32(1.5 * MinWStringCap)
+	nn := b.ResizeTo(n)
+	if len(b.b) != int(nn) {
+		t.Fatalf("resized buffer should be %d, was %d", nn, len(b.b))
+	}
+	if n > nn {
+		t.Fatalf("resized to a value smaller than requested")
+	}
+}
+
+func Test_BufferFree(t *testing.T) {
+	// make sure free-ing doesn't set pooled buffer to nil as well
+	for i := 0; i < 256; i++ {
+		// try allocating and freeing repeatedly since pool does not guarantee item reuse
+		b := NewWString()
+		b.Free()
+		if b.b != nil {
+			t.Fatalf("freed buffer is not nil")
+		}
+
+		b = NewWString()
+		c := b.Cap()
+		if c < MinWStringCap {
+			t.Fatalf("expected capacity >= %d, got %d", MinWStringCap, c)
+		}
+	}
+}