Improve low free memory reporting

[ribbons]

A couple of small enhancements to the memory usage report at the end of a successful compile:

• Send the low available memory warning to err rather than out so that it appears as a more visible warning.
• Remember the size of the text and data from the last compile and show how much of a change there was (if any).

[ribbons]

@cmaglie Would it be possible for you to take a quick look at this when time permits?

[ribbons]

Is there a problem with what I'm proposing in this PR or the way I've implemented it, or is it just a case of waiting until it reaches the top of the pile?

[cmaglie]

Hi @ribbons!
I'm really sorry for the delay.

These two lines of text are the most "disputed" in the history of Arduino, currently they are the result of a ton of discussions and mediation between two opposite designs: a lot of information but terse and potentially confusing (for the beginner) vs. concise information but clearly explained.
Let me check this patch with the developers list to see if there are concerns (or maybe you want to do it?).

[ribbons]

Hi @cmaglie, as the general agreement on the developers mailing list was that the memory usage delta display wasn't really of use, I've dropped that commit out of this PR.

This just leaves the commit which fixes a whitespace error and sends the low available memory warning to err instead of out (causing it to appear in orange instead of white text in the IDE). Would that be something that you would consider merging?

[cmaglie]

Yup, thanks.