rework std.base64 api

* 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

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