Multiple FORMATs merge instead of last-one-wins

[teunbrand]

This PR implements suggestion here: #558 (comment)
@thomasp85 if you want this now rather than later, feel free to merge

[thomasp85]

In general this looks good, but I do have some concerns over how defaults are handled during overrides. For the column type specifically we would have

FORMAT STUB price ...
FORMAT price 
  SETTING hjust = "left"

retarget price to BODY where the user expectation is probably to not change the type unless specified

Something akin to this is happening in renaming because a default value is deduced from the content ({}). That means that

FORMAT price 
  RENAMING * => "{:num %.2f}"
FORMAT price 
  RENAMING * => "{}"

will be a no-op. Probably an edge case but super surprising if encountered.

The better approach is for both structs to encode the content as an Option<> value

[teunbrand]

For this piece:

FORMAT STUB price ...
FORMAT price 
  SETTING hjust = "left"

I'd rather require FORMAT BODY every time (make target non-optional) than having a persnickety 'the default is body but we won't enforce it if a previous clause has already declared that column to be a stub'. It smells too much of magic defaults to me.

For this piece:

FORMAT price 
  RENAMING * => "{:num %.2f}"
FORMAT price 
  RENAMING * => "{}"

Yeah I agree

[thomasp85]

hmm... I think I disagree... The default is on the column, not on the query. So FORMAT col when lacking a type will ask the column what type it is and the default will be BODY.

However, if we have already given the column a type from a prior FORMAT clause then the column will respond with that

This is not how it is directly implemented, I know, but that is how I think about it

[teunbrand]

The solution I've taken is to have the format.target and format.value_template (the *-wildcard renaming entry) be Options. None will not override previous instructions and will be overridden by later instructions. For targets, the default is now FORMAT ANY, which will be parsed to format.target = None.

[teunbrand]

I'm now also allowing FORMAT * to target all columns, convenient when you want to FORMAT * RENAMING null => '' or something.