sprintf implementation

[29jm]

There's a printf function, but no sprintf, snprintf etc... which are really useful, and used in most C apps (including DOOM).

Integrating stb_printf in SnowflakeOS seems like a good idea: it's fully-featured and extensively tested (compared to anything we could hand-roll).

Some pointers:

• copy the header to 'libc/include/deps'
• create a corresponding source file that includes it in "libc/src/stdio", defining STB_SPRINTF_IMPLEMENTATION as needed, along with other symbols that control the functions included
• include the header from "stdio.h", with the same flags except for STB_SPRINTF_IMPLEMENTATION
• test the new functions!

[29jm]

Closed by 3b55ee5bd97c606feb61457edd9bc0cdba67cc61 !