Refactor CleanupStr in misc/pstring.cpp

[Nakhr11n]

Depends on #197.

[Nakhr11n]

Here are some benchmarks:

Benchmark -O3	Time	CPU	Iterations
Original	217 ns	217 ns	3231907
New	85.7 ns	85.3 ns	8203981

Benchmark -O0	Time	CPU	Iterations
Original	278 ns	276 ns	2511607
New	98.5 ns	97.7 ns	7093489

[winterheart]

I'd vote for my C implementation as it faster no matter of what, but let's hear what others thinks.

[winterheart]

I've tested changes and all OK, thanks!