- Missing letter "w" and "W" in list of characters meant only 62
  letters, so less than 6 bits of randomness per salt char.
- A char array like this one still has a trailing NUL char which
  affects the sizeof. Since bounded produces a number < its second
  argument, need to discount the NUL so we only index into
  the letters-part of the string.

Add static_assert() to catch this.