Cleanup some option related code

[michael-schwarz]

Using map_default, map_default_delayed, and friends makes the code much more concise.

[sim642]

This simplifies some uses but I'm unsure about others.
In particular, replacing some matches with large bodies with Option.map_default (or similar) sometimes moves what's being matched much further down (after the lambda). This might hurt readability because the code doesn't read top-down as it executes.

I think it would be good for someone else to also have a look with this in mind to see if they also get this impression.

[michael-schwarz]

What is our plan here? Which ones do we want to merge and what do we want to close?

[arkocal]

Apart from Simmo's open questions I would take this as is, it is in general a good improvement.

Out-of-scope for this PR, but somehow related:
The parts where I have most difficulty reading are large branches in match statements of large functions in Option functions. Without knowing the logic well they are pretty much impossible to read either way (match etc vs. option). I would much prefer functions with descriptive names over lambdas in those cases.

• this allows us to see the map and the default value (or similar) at one glance.