* This is a string type which heavily optimises for immutability and minimal
storage. It only contains one pointer to the string data and always
reallocates on modify. For compile-time literals it doesn't modify or
allocate.
* On x64 we use the top bit in a tagged pointer to store a flag of whether it's
heap or literal, on other platforms it uses a separate field (meaning another
pointer sized value effectively, including padding).
* This is best for structured data which tends to use a lot of immutable strings
for type/name information, and only a few for actual string data (which are
only allocated once and aren't modified after that). Similarly we rarely want
to know only the size of any of these strings, we want the whole string so not
explicitly storing the size is not a big deal.
* Overall this reduces SDObject from 128 bytes to 80 bytes.
* We have some special handling to allow SWIG wrapping of these types:
SDFile owns the chunks and buffers within, and each object owns its
children. Copying is disallowed except from SWIG where we assume the
wrapper is handling lifetime management for its objects externally.
baldurk