Update 0840-no-panic-in-c-string.md

[llogiq]

Added an alternative design which renames the currently used functions.

[llogiq]

Rationale: I think the panicky functions are easier to use if the programmer knows that the strings contain no Null chars, so keeping them, but renaming them to show that they unpack the slice/vec seems a superior alternative.

[alexcrichton]

Thanks for the extra alternative!