Fix console formatting with colors

[AltGr]

closes #174

[AltGr]

This should do it, at least for the main printers. A few remarks:

• the trick is to use either pp_print_as to specify the length in screen columns, when adding the color escapes; alternatively you can use @<n> if you statically know the length to print (e.g. let pp_red_escape fmt = Format.fprintf fmt "@<0>%s" "\027[31m".
• Many code paths go to strings before printing (Format.asprintf) so the trick won't work there; I assumed that this cases correspond to cases where you don't need formatting anyway.
• I noticed many cases of Format.fprintf fmt "%a" f x, which is more simply written f fmt x.
• If you are interested it's possible to define specific tags in Format. Probably some libs do this already... handling nesting properly is tricky.

[denismerigoux]

It works, it's so satisfying! Plus the fix was relatively minor, congrats @AltGr!

[denismerigoux]

Can you deprecate and remove this one?

[AltGr]

Well it's still used in many places where printing doesn't go through Format, I think it can stay

[denismerigoux]

Many code paths go to strings before printing (Format.asprintf) so the trick won't work there; I assumed that this cases correspond to cases where you don't need formatting anyway.

I'm afraid that a lot of these paths that use a string before printing are because of me unable to use Format correctly...

[AltGr]

Well Format is pretty good for formatting code, but has some inflexibilities that make it not a silver bullet for printing all kinds of messages, I am not among the ones that want everything to go through it everywhere (although as you noticed, mixing Format and non-Format printers for all datatypes soon becomes cumbersome). For example, printing a table with Format is basically impossible.