Base64::encode : const correctness / String by reference passing

[dirkmueller]

This is another instance in the core library where we pass in read-only
parameters as pass-by-value, where in the case of String() that
is inefficient as it involves copy-constructor/temp string creations.

[earlephilhower]

These changes LGTM.

The actual base64::encode() function is kind of awful. Figure out how large the result will be, allocate it, write the encoded data to the buffer, and then create another string of the same size (malloc 2x) and memcpy the result, then free fir first buffer.

But that's another PR...

[dirkmueller]

@earlephilhower Inoticed the same, but I failed so far to "use" the String buffer for this. Only trick I know is with std::move().

[earlephilhower]

I think you'd need to make class base64 a friend to get access to the String::wbuffer() (aka writable buffer pointer) method (and String::reserve() the proper length first, of course). Otherwise some sort of new String::assumeBuffer() or something would need to be added. Either would engender more debate than this PR, so I'd worry about it later.

[earlephilhower]

@dirkmueller I realized you can actually do inline base64 encoding directly into the string w/o any add'l String accessor methods. Just pass in the String and instead of writing bytes to the array, just += the chars as they're generated. Easy-peasy, and 1/2 the RAM needed. But that's another PR. :)

[devyte]

Don't forget to reserve() beforehand.