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 From your implementation it seems that the invariant is no longer valid isn't?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 From your implementation it seems that the invariant is no longer valid isn't?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.