core/fmt: add nprint family of procs to get required buffer sizes

[mailgerigk]

This pr adds nprint* style procs to fmt to be easily query how many bytes are needed to store the formatted output. This is useful if you want to acquire a buffer with the correct size to use with the bprint* procs.

[gingerBill]

I see no need for this to exist in fmt but rather you want to create a stream that just counts the number of bytes written and then pass it to the fmt.wprint* procedures.