feat(c/libuv/demo): Add libuv async_fs demo

Signed-off-by: hackerchai <i@hackerchai.com>

fix(c/libuv): fix fs demo

Signed-off-by: hackerchai <i@hackerchai.com>

refactor(c/libuv): neat comment and adapt merge

Signed-off-by: hackerchai <i@hackerchai.com>

c/libuv/_demo/libuv/fs.go

@@ -0,0 +1,70 @@
+package main
+
+import (
+	"github.com/goplus/llgo/c"
+	"github.com/goplus/llgo/c/libuv"
+	"github.com/goplus/llgo/c/os"
+	"golang.org/x/tools/container/intsets"
+	"unsafe"
+)
+
+const bufferSize = 1024
+
+var buffer []c.Char
+var iov libuv.Buf
+
+var loop *libuv.Loop
+var readReq *libuv.Fs
+var closeReq *libuv.Fs
+var openReq *libuv.Fs
+
+func initBuffer() {
+	buffer = make([]c.Char, bufferSize)
+}
+
+func main() {
+	loop = libuv.DefaultLoop()
+	initBuffer()
+
+	file := libuv.NewFile(loop, openReq)
+
+	path := c.Str("example.txt")
+
+	file.Open(path, os.O_RDONLY, 0, onOpen)
+
+	loop.Run(libuv.RUN_DEFAULT)
+
+	libuv.FsReqCleanup(openReq)
+	loop.Close()
+	c.Free(unsafe.Pointer(loop))
+}
+
+func onOpen(req *libuv.Fs) {
+	if req.GetResult() < 0 {
+		c.Fprintf(c.Stderr, c.Str("Error opening file: %s\n"), libuv.Strerror(req.GetResult()))
+	} else {
+		iov = libuv.InitBuf(unsafe.SliceData(buffer), c.Uint(unsafe.Sizeof(buffer)))
+		libuv.FsRead(loop, readReq, req.GetResult(), []libuv.Buf{iov}, 1, -1, onRead)
+	}
+	libuv.FsReqCleanup(req)
+}
+
+func onRead(req *libuv.Fs) {
+	if req.GetResult() < 0 {
+		c.Fprintf(c.Stderr, c.Str("Read error: %s\n"), libuv.Strerror(req.GetResult()))
+	} else if req.GetResult() == 0 {
+		libuv.FsClose(loop, closeReq, req.GetResult(), onClose)
+	} else {
+		if req.GetResult() > intsets.MaxInt {
+			c.Fprintf(c.Stderr, c.Str("Too big file.\n"))
+		}
+		c.Printf(c.Str("%.*s"), c.Int(req.GetResult()), buffer)
+		libuv.FsRead(loop, req, req.GetResult(), []libuv.Buf{iov}, 1, -1, onRead)
+	}
+	libuv.FsReqCleanup(req)
+}
+
+func onClose(req *libuv.Fs) {
+	c.Printf(c.Str("\nFile closed.\n"))
+	libuv.FsReqCleanup(req)
+}