Add support for `strspn` and `strcspn`

[sosthene-nitrokey]

Hi!

We are developing the bindings for the C library littlefs (littlefs2-sys), and we have to include implementations of strspn and strcspn that are used by littlefs.

A PR made us aware of tinyrlibc, where it would seem to make more sense to have these implementations. Would these fit here?

[Sympatron]

I think it would fit very well.
My suggested implementation would be like this:

#[no_mangle]
pub unsafe extern "C" fn strcspn(str1: *const CChar, str2: *const CChar) -> CSizeT {
    let mut count = 0;
    let mut s = str1;

    while *s != 0 {
        if !strchr(str2, *s).is_null() {
            break;
        }
        count += 1;
        s = s.add(1);
    }

    count
}
#[no_mangle]
pub unsafe extern "C" fn strspn(str1: *const CChar, str2: *const CChar) -> CSizeT {
    let mut count = 0;
    let mut s = str1;

    while *s != 0 {
        if strchr(str2, *s).is_null() {
            break;
        }
        count += 1;
        s = s.add(1);
    }

    count
}

[jonathanpallant]

I'll work on this