reject HashMap, embrace match

// This should be generated by charset_index.py
fn charset_index(x: &char) -> Sample {
    match x {
        '0' => 0.0,
        '1' => 0.0002366863905325444,
        '2' => 0.0009467455621301776,
        '3' => 0.0021301775147928997,
        '4' => 0.0037869822485207105,
        '5' => 0.00591715976331361,
        '6' => 0.008520710059171599,
        '7' => 0.011597633136094675,
        '8' => 0.015147928994082842,
        '9' => 0.019171597633136098,
        'A' => 0.02366863905325444,
        'B' => 0.028639053254437875,
        'C' => 0.034082840236686396,
        'D' => 0.04000000000000001,
        'E' => 0.0463905325443787,
        'F' => 0.053254437869822494,
        'G' => 0.06059171597633137,
        'H' => 0.06840236686390533,
        'I' => 0.07668639053254439,
        'J' => 0.08544378698224853,
        'K' => 0.09467455621301776,
        'L' => 0.10437869822485209,
        'M' => 0.1145562130177515,
        'N' => 0.125207100591716,
        'O' => 0.13633136094674558,
        'P' => 0.14792899408284024,
        'Q' => 0.16000000000000003,
        'R' => 0.17254437869822487,
        'S' => 0.1855621301775148,
        'T' => 0.19905325443786984,
        'U' => 0.21301775147928997,
        'V' => 0.22745562130177518,
        'W' => 0.24236686390532547,
        'X' => 0.2577514792899408,
        'Y' => 0.2736094674556213,
        'Z' => 0.28994082840236685,
        '`' => 0.30674556213017756,
        '-' => 0.32402366863905324,
        '=' => 0.3417751479289941,
        '[' => 0.36,
        ']' => 0.37869822485207105,
        ';' => 0.3978698224852071,
        '\'' => 0.41751479289940835,
        ',' => 0.4376331360946745,
        '.' => 0.458224852071006,
        '/' => 0.4792899408284023,
        '~' => 0.500828402366864,
        '!' => 0.5228402366863905,
        '@' => 0.5453254437869823,
        '#' => 0.5682840236686391,
        '$' => 0.591715976331361,
        '%' => 0.615621301775148,
        '^' => 0.6400000000000001,
        '&' => 0.6648520710059171,
        '*' => 0.6901775147928995,
        '(' => 0.7159763313609467,
        ')' => 0.7422485207100592,
        '_' => 0.7689940828402366,
        '+' => 0.7962130177514793,
        '{' => 0.8239053254437869,
        '}' => 0.8520710059171599,
        '|' => 0.8807100591715976,
        ':' => 0.9098224852071007,
        '<' => 0.9394082840236686,
        '>' => 0.9694674556213019,
        '?' => 1.0,
        _ => panic!("{}", format!("FIXME: char {:#?} not indexed:", x)),
    }
}