Bigger features and improvements

[ ] Preserve some whitespace (hard to read lots of long expressions bound in the imperative fragment), see #1086
[x] Preserve comments, see #1086 #1172
[x] #1216
[ ] VS Code plugin support (if there is any special need), see #788
[ ] Port pretty-printing randomized tests from scilla-chick to qcheck: we don't need to generate well-typed programs to test the formatter. This will also increase test coverage.

Small and/or easy improvements

[ ] Support for Scilla libraries (.scillib files)
[ ] A CLI option to set the width of the formatting window
[ ] CLI: return specific error codes for different types of failures (other than CLI argument parsing errors)
[ ] Generate man pages for the scilla-fmt binary
[ ] Format imports on one line if these do not contain namespaces
[ ] Less parentheses for types
[ ] Special treatment for function of multiple arguments: do not increase indentation for these
[ ] Special treatment for
```
let foo =
let bar = ... in ...
in
...
```
The second let for short expressions ends up on the same line as the first one, which hinders readability:
```
let foo = let bar = ... in ...  in ...
```

[ ] Type definitions can be pretty unreadable with contract address types:

type Foo =
| Foo of ByStr20 with contract ............................. end Uint256