Support Uchar.t as testable

[emengd]

Hi, I propose this tiny addition of a testable for Uchar.t values.

There is a non-obvious question of how to print such values. Currently the stdlib has neither a formatter for uchars nor a literal syntax to read them back (see upstream discussion here), so it is not possible to provide a directly useable value. I am reusing Fmt.Dump.uchar which does Fmt.pf ppf "U+%04X" (Uchar.to_int uc), for instance "é" becomes U+00E9. To read it back (for instance in the toplevel) you need to write Uchar.of_int 0x00E9.

Some options:

1. Enclose it in quotes to make it clear it's one value
2. Use the syntax used in string literals, e.g. "\u{00E9}" (this is a valid string value)
3. Special case ASCII and print it using %C

But I'm not convinced by any of these, if you're using uchar's you're probably doing something fairly technical and a pure code is good enough for debugging.