Implement simple compression / encoding of meterpreter stages

[busterb]

This is especially evident in x64 mettle payloads, but because the payloads include padding, the elf2bin payload blobs are a lot larger than they need to be. A 700k elf file becomes almost 3MB on the wire because we literally transfer all of the padding null bytes into the target memory image. This is a similar story with Python meterpreter where quite a bit is whitespace.

It would be nice if there was some simple compression/encoding stagers could implement to transfer stages in a smaller way. Zlib might be too big, but even a RLE (Run Length Encoding) transfer would cut down on size substantially. If we're ok with a C-based stager, there are probably a lot more things we could try.

[itsmeroy2012]

The stagers in the python payloads are base64 encoded. Can we implement the same in case of the stages because base64 encoding is provided as a direct function in python. @busterb @zeroSteiner

[busterb]

We shrunk the null pages, but it's not nearly as bad as it was. Definitely improved enough for my initial complaint.