Add a way to create U32/U16 string from number

[playXE]

I'm using widestring to implement R7RS/R6RS Scheme VM and to convert number to string I have first to use to_string that creates String and after this I use U32String::from_str which does 2 heap allocations.

[starkat99]

Added std::fmt::Write trait implementation for UString. This means you'll be able to do write!(ustr, "{}", 1234) and avoid the heap allocations, or for any other string formatting.