Clean up use of sprintf

In #208 I added pragmas to suppress deprecation warnings related to sprintf that were added in newer versions of GCC and CLang, and which caused compilation failure. These suppressions should be removed.

One usage is in the test suite's implemention of `itoa` that is needed to run String tests. I have a quick fix for that relying on `snprintf` but I also thought of just using a more basic implementation of `itoa` such as the one from K&R or even just copying the implementation from the SAMD core.

Thinking a bit deeper though, the only use in API code is in `String` and it looks to be unnecessarily double buffering during those conversions. I think there's an opportunity to remove String's dependency on `itoa` and make it more efficient, which would also eliminate the need for an implementation of `itoa` in the test suite. I plan to look in to that approach.

A similar use is in the test suite's implementation of `dtostrf` which delegates the deprecated avr impl. For that, I think `String` can be improved to avoid the double buffering although it would still delegate to the core for the conversion, assuming the core may have implementation-specific FP support.

The final use was in `IPAddress` and that looks like it can simply be changed to used `snprintf`

Locate the String and IPAddress API implementations and the test suite's itoa and dtostrf helpers. Read the existing sprintf pragmas and conversion paths first. Done means removing the suppressions and addressing the identified deprecated sprintf uses without breaking String, IPAddress, or String tests.

Written by the indexing model from the issue text.