Codelens eval should not interact with IO

### Subject of the issue
`-- >>> getChar` actually returns a value. It seems to be reading the characters from the link between the server and editor. However it does not seem to cause any crashes or ruin the link, on my editor at least. `putStr` etc. might also possibly be doing something unexpected although it does not seem to produce any noticeable effects on the editor.

### Your environment
`neovim 4.4`, `coc.nvim`.

The codelens eval should perhaps perform some pre-processing and get rid of any IO first, just to be safe.