* rename decode to decodeExactUnsafe.
* add decodeExact, which checks for invalid chars and padding.
* add decodeWithIgnore, which also allows ignoring chars.
* alphabets are supplied to the decoders with their
char-to-index mapping already built, which enables it to be
done at comptime.
* all decode/encode apis except decodeWithIgnore require dest
to be the exactly correct length. This is calculated by a
calc function corresponding to each api. These apis no longer
return the dest parameter.
* for decodeWithIgnore, an exact size cannot be known a priori.
Instead, a calc function gives an upperbound, and a runtime
error is returned in case of overflow. decodeWithIgnore
returns the number of bytes written to dest.
closes#611
