fuzz testing?

[mschoch]

Seems a reasonable candidate. Two main challenges are:

a) at the build state, keys must be inserted in order, so using fuzz data as input isn't straightfoward
b) at runtime we don't do much/any bounds checking on the FST, so we surely panic on this today

Regarding challenge b, see #1 which might offer a safe-mode. In this mode we should return error and not panic.