Support writing ELF files too

[Wilfred]

I'd like to be able to write ELF files. Would it be possible to add this functionality to rust-elf? With all the struct definitions, I almost wonder whether this is already possible.

[aep]

Fwiw this seems to support writing and appears to be popular: https://github.com/m4b/goblin

Haven't tried since I wrote https://github.com/aep/elfkit for manipulating elf