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Idiom #216 Pad a string in the center
Prepend extra character c at the beginning and ending of string s to make sure its length is at least m.
After the padding the original content of s should be at the center of the result.
The length is the number of characters, not the number of bytes.
E.g. the with s = "abcd", m=10 and c="X" the result should be "XXXabcdXXX".