New implementation

[AnttiParaoanu]

New implementation works as follows:

• Store the discriminant bit in the most significant bit of SmartString
• Arrange the fields of ´BoxedString´ so that the discriminant bit is the most significant bit of capacity or size
  • If on a little endian arch and lazy_null_pointer_optimizations is disabled, use size, otherwise capacity
• Since transmuting into a String is not guranteed to work, boxed string methods use a new struct called StringReference, which contains a reference to a SmartString, and a String. At the end of it's lifetime the contents of the String are transferred to the SmartString. This replaces try_demote

Null pointer optimizations:

• If the string is inlined, the discriminant bit is set, so the usize it is a part of (in the boxed version) is not zero
• If the string is boxed, the capacity is not zero
• If the string is boxed, and the mode is Compact, the size is also not zero

Things to do:

• Fix bug smartstring breaks std::string::String? #7
• More tests?
• Decide if lazy_null_pointer_optimizations should be a flag or not (maybe not tbh)

[bodil]

OK, it's going to take me a while to read through all this, but it looks very interesting.

If nothing else, the lazy_null_pointer_optimizations feature flag needs to be shortened, it's a bit of a mouthful, but I'd like to benchmark it a bit to see if it could just be removed.