Can be performance problem reported by clippy::large-enum-variant

[olexiyb]

https://stackoverflow.com/questions/51200405/solutions-to-performance-issues-caused-by-large-variant-size-of-enum

• hook id: clippy

• exit code: 101

  Checking actors v0.1.0 (/home/olexiyb/b100pro/actors.rs)
  error: large size difference between variants
  --> src/kernel/mod.rs:28:5
  |
  28 | Sys(ActorSystem),
  | ^^^^^^^^^^^^^^^^
  |
  = note: -D clippy::large-enum-variant implied by -D warnings
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant
  help: consider boxing the large fields to reduce the total size of the enum
  |
  28 | Sys(Box),
  | ^^^^^^^^^^^^^^^^