atchops_base64_encode: return null-terminated buffer

[XavierChanth]

atchops_base64_encode requires a pre-allocated buffer, which is an anti-pattern for returning null-terminated strings. We can coerce our own null-terminated string by allocating one byte longer than we pass as the buffer size into this function. The style for this function should change to not require input or output parameters, and the function should determine and allocate the buffer itself.

[XavierChanth]

We have a function called atchops_base64_encoded_size which provides the size required for a buffer to be encoded in base64. If you pass that size in to atchops_base64_encode you will get a buffer which is not null-terminated, but if you pass a size which is larger than that by at least one byte, then you will get a null-terminated buffer.

I believe this to be the best of both worlds.