String: informative messages on issues

[d-a-v]

• Always enable messages when serial debug port is enabled
• Allocation issue was not reported

fixes #8779

Messages are like:

12:14:33.749616204 [String] 'ab1ab2ab3a ... 08ab8309ab': Reallocating large String(48749 -> 48753 bytes)
12:14:33.758096639 [String] 'ab1ab2ab3a ... 11ab8312ab': Reallocating large String(48767 -> 48771 bytes)
12:14:33.766779262 [String] 'ab1ab2ab3a ... 8314ab8315': Reallocating large String(48783 -> 48785 bytes)
12:14:33.774862098 [String] 'ab1ab2ab3a ... 16ab8317ab': Reallocating large String(48797 -> 48801 bytes)
12:14:33.780903383 [String] 'ab1ab2ab3a ... 16ab8317ab': OOM: 48799 -> 48816 bytes
12:14:33.789079777 [String] 'ab1ab2ab3a ... ab8317abab': Reallocating large String(48799 -> 48803 bytes)
12:14:33.797860839 [String] 'ab1ab2ab3a ... ab8317abab': OOM: 48799 -> 48816 bytes
12:14:33.806152000 [String] 'ab1ab2ab3a ... ab8317abab': Reallocating large String(48799 -> 48801 bytes)

from code:

int cnt = 0;
String puffed;

void loop() {
    puffed += "ab";
    puffed += String(++cnt);
}