Port exec command (#849)

* Port exec command

* fix windows

* lint

crates/nu-command/src/system/mod.rs

@@ -1,10 +1,12 @@
 mod benchmark;
+mod exec;
 mod ps;
 mod run_external;
 mod sys;
 mod which_;
 
 pub use benchmark::Benchmark;
+pub use exec::Exec;
 pub use ps::Ps;
 pub use run_external::{External, ExternalCommand};
 pub use sys::Sys;