js: roman-numerals: exact-match shortcut

if roman_in_place() sees a value that is specified in the
DECIMAL_TO_ROMAN mapping, it shortcuts the switch and returns it

this is slightly faster when running the test suite 3981 times:

[without shortcut]
    Finished in 8.899 seconds

    real    0m9.680s
    user    0m5.339s
    sys     0m0.321s

[with shortcut]
    Finished in 8.618 seconds

    real    0m9.395s
    user    0m5.314s
    sys     0m0.318s