std: fold into embassy core, add non-hacky time driver.

examples/std/src/bin/net.rs

@@ -2,11 +2,10 @@
 #![allow(incomplete_features)]
 
 use clap::{AppSettings, Clap};
-use embassy::executor::Spawner;
+use embassy::executor::{Executor, Spawner};
 use embassy::io::AsyncWriteExt;
 use embassy::util::Forever;
 use embassy_net::*;
-use embassy_std::Executor;
 use heapless::Vec;
 use log::*;