... and add a custom printer
Since this is a very common bug, this patch should gain us a lot of time when
debugging uncaught Not_found errors, because the element not found can now be
printed straight away without the need for further debugging.
The small cost is that one should remember to catch the correct specialised
`Foo.Map.Not_found _` exception rather than the standard `Not_found` (which
would type-check but not catch the exception). Using `find_opt` should be
preferred anyway.
Note that the other functions from the module `Map` that raise `Not_found` are
not affected ; these functions are `choose`, `min/max_binding`,
`find_first/last` which either take a predicate or fail on the empty map, so it
wouldn't make sense for them (and we probably don't use them much).
This patch functorises the generic expression printer, in order to be able to
re-use it for end-user printing.
It makes it possible to have an end-user, localised printer that shares the code
for e.g. priority and automatic parens handling.
A generic AST rewriting that disambiguates variables (very simple to write with
bindlib) is also added and used in the OCaml backend for something safer than
just appending `_user` (-- this also handles clashing variables that could be
introduced during compilation which would have generated wrong code before this)
Finally, the `explain` plugin is adapted to use the new printer.
Ah, and `String.format_t` was tweaked to correctly print strings that might
contain unicode without breaking alignment, and should be used instead of
`format_string` or `%s` whenever unicode can be expected.
