Emit inline strncmp() for str IO vmc codegen

[katef]

This is equivalent to #430 but with strncmp() for FSM_IO_STR.

Coverage for the string API is poor. I don't want to try to run the pcre suite for that, because FSM_IO_STR has no provision for binary data, being a \0-terminated string. So I've extended how we run retest for a few handwritten test cases instead.

The generated code looks like this:

; ./build/bin/re -k str -l vmc '^abc[xy]'
int
fsm_main(const char *s)
{
    const char *p;
    int c;

    p = s;
    if (0 != strncmp(p, "abc", 3)) return -1;
    p += 3;

    if (c = (unsigned char) *p++, c == '\0') return -1;
    if (c <= 'w') return -1;
    if (c > 'y') return -1;
    return 0x1; /* "^abc[xy]" */
}