Safer/richer indentation abstraction

[olafurpg]

The indentation is managed by State.indent: Int. Splits and policies can increment/decrement the int.

• This makes it tricky for example to indent a function argument to the same column as the opening ( because we don't know the exact integer value of the state's column.
• unindenting the same number of spaces at the closing ) requires us to know integer value at the opening )
• We violate DRY by relying on decrementing the same number as we increment.

A more powerful abstraction would work like a stack, you push an Indent which can be a concrete number or a useful object like state.column

[olafurpg]

Closed by #31