Correct salting array:

• 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.