To parse version numbers written semver-style.

§1. The notable thing about this code is that we could match v4, v5, and so forth, but choose not to: we want to avoid collisions with variable names innocently chosen as "v" plus a number. So you need to use at least one dot. "v4.0" achieves the same effect as "v4" would in any case.

<s-literal-version-number> internal
    if (Wordings::length(W) != 1) return FALSE;
    inchar32_t *p = Lexer::word_raw_text(Wordings::first_wn(W));
    if ((p) && (p[0] == 'v')) {
        int segments[3] = { 0, 0, 0 };
        int segment_count = 0, digits = 0;

        int i = 1;
        while (p[i]) {
            if (Characters::isdigit(p[i])) {
                segments[segment_count] = 10*segments[segment_count] + ((int) p[i] - (int) '0');
                digits++;
                if (digits > 9) {
                    ==> { fail nonterminal };
                }
            } else if ((p[i] == '.') && (segment_count < 2) && (digits > 0)) {
                digits = 0; segment_count++;
            } else {
                ==> { fail nonterminal };
            }
            i++;
        }
        if ((digits > 0) && (segment_count > 0)) {
            TEMPORARY_TEXT(vtext)
            WRITE_TO(vtext, "%d", segments[0]);
            if (segment_count >= 1) WRITE_TO(vtext, ".%d", segments[1]);
            if (segment_count >= 2) WRITE_TO(vtext, ".%d", segments[2]);
            semantic_version_number V = VersionNumbers::from_text(vtext);
            DISCARD_TEXT(vtext)
            ==> { -, Rvalues::from_version(V, W) }
            return TRUE;
        }
    }
    ==> { fail nonterminal };
}