Every single call to base64decode allocates a mapping table for all
base64 characters. This is quite wasteful, as the map is in fact static.
We could use a static variable here, but that would have unpleasant consequences
if we ever encounter input with non-valid base64 characters (which
implicitly modifies the map).
The number of character ranges for base64 is quite low (3, plus 4 exceptions),
thus we can simply check that explicitly in code instead of using a dynamic hash table.