Remove duplicated code for value pretty-printers

[shym]

Lin and STM both use 'a -> string functions in their main type to display values but they each redefine their set of functions for common types. All this machinery should be shared and so moved to Util.
Util.Pp introduced in #342 would be a good candidate for that.
Beside sharing the code, the 'a -> string type could be replaced with 'a Util.Pp.t to get prettier outputs (ie with parentheses when needed) if we deem it not to break too much user code.