Speed up ELF symbol lookup

[wannacu]

It's too slow to rebuild an ELF object that contains a large number of symbols. This happens because the frequent linear looking up was expensive.

[romainthomas]

Hi @wannacu

Thank you for this interesting PR!.
Some remarks though:

1. It uses std::string_view in public headers which raises the minimum C++ version from 11 to 17. I would like to stick to C++11 for the API
2. What if a user add a symbol? From your implementation it seems that the invariant is no longer valid isn't?

[wannacu]

Hi @wannacu

Thank you for this interesting PR!. Some remarks though:

1. It uses std::string_view in public headers which raises the minimum C++ version from 11 to 17. I would like to stick to C++11 for the API
2. What if a user add a symbol? From your implementation it seems that the invariant is no longer valid isn't?

Thank you for your comment

1. It is also acceptable to use std::string
2. If the user adds a valid symbol with a unique name, the corresponding key will also be valid.

[peledins-zimperium]

This is a very important fix and would enable removing symbols en-mass from LIEF.
However, there is a corner case not handled: there can be 2 symbols that go by same name, for example, when same static function is defined in 2 source files, e.g.:

objdump --syms ./a.out 

./a.out:	file format mach-o arm64

SYMBOL TABLE:
0000000100003f70 l     F __TEXT,__text __ZL3barv
0000000100003f98 l     F __TEXT,__text __ZL3barv
0000000100003f84 g     F __TEXT,__text __Z3foov
0000000100000000 g     F __TEXT,__text __mh_execute_header
0000000100003f50 g     F __TEXT,__text _main
0000000100004000 g     O __DATA,__common _z

bar() is static.

[peledins-zimperium]

Oh, and the fix is for ELF only... similar should exist for MachO... and my example above is for machO.

[peledins-zimperium]

And it is a case for ELF: https://stackoverflow.com/questions/71408568/elf-symbol-name-occurs-twice-data-rel-ro-and-bss