New fixity/iterator syntax

[janmasrovira]

• Closes Syntax change proposal for fixity declarations #2330
• Closes Fixity yaml not visible enough in the html source using a dark theme #2329

This pr implements the syntax changes described in #2330. It drops support for the old yaml-based syntax.
Some valid examples:

syntax iterator for {init := 1; range := 1};

syntax fixity cons := binary {assoc := right};
syntax fixity cmp := binary;
syntax fixity cmp := binary {}; -- debatable whether we want to accept empty {} or not. I think we should

Future work

This pr creates an asymmetry between iterators and operators definitions. Iterators definition do not require a constructor. We could add it to make it homogeneous, but it looks a bit redundant:

syntax iterator for := mkIterator {init := 1; range := 1};

We could consider merging iterator and fixity declarations with this alternative syntax.

syntax XXX for := iterator {init := 1; range := 1};

syntax XXX cons := binary {assoc := right};

where XXX is a common keyword. Suggestion by @lukaszcz XXX = declare

[lukaszcz]

The syntactical asymmetry between iterator and fixity is justified because fixity declares a new idenfitier and assigns fixity to it, while iterator declares an already existing identifier to be an iterator.

[lukaszcz]

This PR contains large deltas with formatting changes putting newlines everywhere. Is this a result of a different version/configuration of ormolu or was this intentional?

Personally, I think inserting these newlines makes things less readable. But even if we decide to change the coding style to insert newlines in these places, we should do it in a separate PR.