Use bytes.HasPrefix for null length detection.

bytes.HasPrefix uses optimized ASM, which is rad.
Avoid redundant checks in parseLenLine.

~16% faster, believe it or not. Add a benchmark, too.
Further improvements to parseLenLine.

Another ~6% speed up and better error checking.
Add documentation.
String improvements.

Add tests, add NewBulkString(), better docs, etc.