Centos7 build fix

[neandreithal]

I've managed to get #507 working with llvm-toolset-7 on CentOS 7. Unfortunately, I can't move to devtoolset-9 as it was done in the original issue.

[danielaparker]

I appreciate the effort!

I'd like to avoid constructing strings with s.cstr(), as that would result in a large number of length recalculations, when we already know the length.

For key_type, you should replace

key_type(s->first.c_str(), get_allocator())

with

key_type(s->first.data(), s->first.size(), get_allocator())

Similarly, you should replace

 keys.emplace(key.c_str(), get_allocator());

with

 keys.emplace(key.data(), key.size(), get_allocator());

[neandreithal]

Done, thanks for the feedback!