Refactor how computation expressions are formatted

[nojaf]

The problem with the current implementation was that the SynExpr.LetOrUseBang could be part of a nested SynExpr.LetOrUse expression and that would mean that the first let bindings inside a computation expression are formatted somewhere else.

That made it hard to determine whether the first let! needed a newline before it.
In my proposed solution, I check whether the expression (inside the CompExpr) contains something related to a bang keyword let!, return!, and!, etc., if that is the case collect all expressions (and especially the let bindings).

Once all expressions are collected, merged them together using the colWithNlnWhenItemIsMultiline helper function.

This fixes #838 and should make Computation Expressions, in general, adhere to the rules when we add newlines between expressions.