Allow single line blocks for imports and enums

[Hubro]

There are some situations where blocks are (almost) always very short and would be more readable on a single line. For example:

import ietf-inet-types { prefix inet; }
import ietf-routing-types { prefix routing; }

enum "foo" { value 10; }
enum "bar" { value 20; }
enum "baz" { value 30; }

There should be an option (for example -s/--single-line-blocks) that allows certain blocks (just import and enum to begin with) to be drawn on a single line, assuming they only have 1 child statement.

In these cases, post_comments should be rendered behind the entire block rather than right after the opening curly brace.