DataModelWrapper::toSnakeCase: Take string_view

[anordal]

This function does not modify its string argument, i.e. can use std::string_view, assuming C++17. This avoids implicit std::string temporaries.

Also fixes a use-after-free of the form:

toSnakeCase(name).c_str()

This aligns the function with the one in SDK, including a couple of unrelated changes:

• Loop variable type: int → size_t (would satisfy gcc -Wsign-compare)
• The call to tolower can be made unconditional due to idempotence.

Implements #93

[anordal]

(I split the commit in 3, no change).

Sorry, Mathias, I see you had some review comment(s), but they went away (dunno why).

[anordal]

Moved to #133: The CI jobs don't start when the PR is from an external repository.