Add attribute points

[rachitnigam]

Add attribute points for components, cell declarations, and port definitions:

import "primitives/core.futil";
component main<"static"=1>(@stable(1) @go_port(1) in: 32) -> (@stable(0) out: 32) {
  cells {
    @precious(1) out = prim std_reg(32);
    le = prim std_le(32);
  }
  wires {
    group cond<"stable"=1> {
      cond[done] = 1'd1;
    }
  }
  control {
  }
}

Fixes #311.