Replace dead overflow checks with assertions and avoid indirect call …

…overhead

We allocate twice the input length, and every input character results in
either 1 or 2 output bytes, so we cannot overflow.

By using an enum, and a switch table (which will be efficiently compiled
into a jump table), we can avoid the pessimistic code generation of the
indirect calls.

With this I get the following runtime for the test script in GH-13466 on
my i7-4790, which is around 1.25x faster.
  Time (mean ± σ):     250.9 ms ±   1.6 ms    [User: 248.4 ms, System: 2.0 ms]
  Range (min … max):   248.9 ms … 254.4 ms    11 runs