Add micro's own lua timer function micro.After()

Directly using Go's time.AfterFunc() from lua is tricky. First, it
requires the lua timer callback to explicitly lock ulua.Lock to prevent
races. Second, it requires the lua timer callback to explicitly redraw
the screen if the callback changes the screen contents (see #2923).

So instead provide micro's own timer API which ensures both
synchronization and redrawing on its own, instead of leaving this burden
to lua code. In fact, its implementation runs the lua timer callback in
the main micro's goroutine (i.e. from micro's perspective it is
synchronous, not asynchronous), so both redrawing and synchronization
are ensured automatically.

Fixes #2923

cmd/micro/initlua.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"log"
+	"time"
 
 	lua "github.com/yuin/gopher-lua"
 	luar "layeh.com/gopher-luar"
@@ -55,6 +56,11 @@ func luaImportMicro() *lua.LTable {
 		return action.Tabs
 	}))
 	ulua.L.SetField(pkg, "Lock", luar.New(ulua.L, &ulua.Lock))
+	ulua.L.SetField(pkg, "After", luar.New(ulua.L, func(t time.Duration, f func()) {
+		time.AfterFunc(t, func() {
+			timerChan <- f
+		})
+	}))
 
 	return pkg
 }