Add safe snprintf rule

[lucic71]

The snprintf safe rule uses the new libcc2rs::format_c function. The rule is:

1. let s = libcc2rs::format_c(&a2.to_rust_string(), va);
2. let b = s.as_bytes();
3. if a1 > 0 {
4.   let n = min(b.len(), a1 - 1);
5.   a0.with_slice_mut(n + 1, ... copy n+1 + NUL byte in a0)
6. }
7. b.len()

Initially, I planned to use format! instead of libcc2rs::format_c at line 1 because that's definitely more idiomatic. However we need to support runtime format strings, for example:

char fmt[] = "%d",
snprintf(buf, n, fmt, 123);

The format! macro in rust does not support runtime format strings. It only works with compile time format strings. For this reason I created libcc2rs::format_c which internally uses the sprintf crate. The role of format_c is to validate the format string, convert each libcc2rs::VaArg to sprintf::Printf then do the actual formatting.