Re-implemented the stout ELF abstraction in terms of ELFIO.
Re-implemented the stout ELF abstraction in terms of ELFIO.

As part of this, we updated the API to include functions written in
snake_case (as is standard for stout). We also removed the 'close()'
call, as the new ELFIO library doesn't require it (as it loads the
file into memory!).

Review: https://reviews.apache.org/r/49562/

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