Add string_view utility functions, strict string->numeric parser

[dbaston]

References #10130, #9953

Primary new feature is cpl::strict_parse<T>, which intends to make it easier to check string arguments for invalid values instead of silently converting to zero. The final commits show some example applications of this.

[rouault]

do we formally give up on C++11 public header compatibility ? I'm not against it, but that should be done more formally than in a PR

[rouault]

there's a risk such API could be mis-used if the argument is a tempory like in
auto t = trim(a_str + "x");
I'd suggest we double them with something like :

inline std::string trim(const std::string &str)
{
    return std::string(trim(std::string_view(str)));
}

inline std::string_view trim(std::string &str)
{
    return trim(std::string_view(str));
}

Applies to all those that return std::string_view

[dbaston]

Good point! Here is a possible alternative, in which trim always returns a consistent type:

inline std::string_view trim(const char* s) { 
   return trim(std::string_view(s)); 
}

std::string_view trim(std::string&&) = delete;