Only parse the last SHT_SYMTAB in Elf::parse()

[Lichtso]

Only parse the last SHT_SYMTAB, because all earlier ones are not used anyway.
This improves the runtime complexity from O(n^2) to O(n),
and makes the parsing more robust against crazy inputs.

[m4b]

This was O(n^2) before because it would parse every symtab and every symtab could have referenced the entire string table, is that correct?

[m4b]

Also I never received that binary which was the original impetus for changing string table to avoid DoS attacks :) would still be interested to see such a thing, can send to my email m4b dot github dot io atsign gmail.com