String utils: add toLower; add string to double; fix interface

[filippobrizzi]

Description

Add a couple of new string utilities:

• Extend stringToInt64 to support also double -> stringTo<T>
• Add toLowerCase
• Update the interface of the other functions to use std::string_view instead of const&.
  • Why? See this article https://quuxplusone.github.io/blog/2021/11/09/pass-string-view-by-value/

[floriantschopp]

couldn't we use absl::string_utils for most of this stuff? eg https://github.com/abseil/abseil-cpp/blob/master/absl/strings/numbers.h

[filippobrizzi]

couldn't we use absl::string_utils for most of this stuff? eg https://github.com/abseil/abseil-cpp/blob/master/absl/strings/numbers.h

Great point, I didn't think of checking! Replaced