Special-case T==char in string.Concat(IEnumerable<T>)

Special-case T==char in string.Concat(IEnumerable<T>)

This allows string.Concat to be used as an efficient mechanism for creating a `string` from an `IEnumerable<char>`.  The JIT specializes the implementation for char vs non-char, so there's minimal impact on the non-char case, and for the char case, we a) avoid creating a string for each individual char, and b) use StringBuilder's fast path for adding individual chars.  This can result in a massive allocation savings for long enumerations (for <= 1 char, there's no difference, but each character after that is an allocation saved), and for more than a few characters can yield up to a 2x increase in throughput.