Replaced std::string with std::string_view and removed excessive copies in cudf::io

[lamarrr]

Description

As part of the improvement effort discussed in #15907, this merge request removes some of the excessive std::string copies and uses std::string_view in place of std::string when the lifetime semantics are clear.

std::string is only replaced in this MR in linear functions and constructors, but not in structs as there's no established ownership or lifetime semantics to guarantee the string_views will not outlive their source.
There were also some cases of excessive copies, i.e. consider:

struct source_info{
source_info(std::string const& s) : str{s}{}

private:
std::string str;
};

In the above example, the string is likely to be allocated twice if a temporary/string-literal is used to construct "s": one for the temporary and one for the copy constructor for str

struct source_info{
source_info(std::string  s) : str{std::move(s)}{}

private:
std::string str;
};

The string is only allocated once in all scenarios.
This also applies to std::vector and is arguably worse as there's no small-vector-optimization (i.e. std::string's small-string-optimization/SSO).

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mhaseeb123]

Nice optimization. General question, many of the strings in the PR (except for paths etc) are very tiny (< 10 chars) which may not impact performance or space otherwise. Should we just leave them as is?

[lamarrr]

Nice optimization. General question, many of the strings in the PR (except for paths etc) are very tiny (< 10 chars) which may not impact performance or space otherwise. Should we just leave them as is?

only a few, most of them aren't tiny (<16 characters) in the general case, regardless, incurring extra allocations even from small strings (>16 && < 1'000) when they aren't needed isn't ideal either as they lead to memory fragmentation for long-running programs.

[kingcrimsontianyu]

Approved. But I still have a different opinion that std::string_view should not be used in associative containers.

[mhaseeb123]

Looks like we have build errors from replacing strings with string_views at certain places!

[mhaseeb123]

Looks like we have build errors from replacing strings with string_views at certain places!

Looks like we are getting some runtime errors in libcudf with this PR as well

[mhaseeb123]

Looks like some python tests are now failing due to extra " chars in some of the output cols

[lamarrr]

/merge