Multi-line comment syntax

[osa1]

Currently we don't have a multi-line comment syntax, but we should.

Some inspirations:

• Haskell uses {- ... -}.

• Lua allows starting a multi-line string literal in single line comments. The comment ends when the literal ends. E.g.

  --[[ this is a multi-line comment
  that ends with the string literal
  ]]

  I actually like this quite a bit, but I'm not sure if we want a multi-line string literal syntax that uses angle brackets. I'd prefer double quotes, for consistency with the single-line literals.

• Most semicolon-and-braces langauges use /* ... */. If we go with this we should make it look similar to our comment syntax with --s. Maybe /- ... -/ or -/ ... /-.

[osa1]

I've implemented #| ... |# syntax.

I would like to experiment with indentation-based comment blocks as well, e.g.

#:  This comment block
    ends with the indented
    block.

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