Safety of staticRead and StaticExec?

[Araq]

@iffy1 There is already logic in the compiler to prevent `writeFile` and 
`staticExec` for "nim check" and "nimsuggest" and it should be easy to extend 
this feature. PRs are welcome, really. (Years passed since I wrote my original 
reply and now I like the idea much more than I did in the past.)