z_string_wrap and z_slice_wrap are not symmetrical

[milyin]

Describe the release item

The z_slice_wrap accepts (ptr,len). The matching function for string is z_string_from_substring which also takes (str,len). And function with similar name z_string_wrap accepts only (str). It's more logical to have both "wrap" functions use the same parameters.

Maybe better to review these names and avoid this ambigious word "wrap". It doesn't give any information that the funciton actually copies data from buffer. The only hint is the resulting type - "owned" - but this is not enough

[Mallets]

Superseded by #548