Store contents of static string struct in the array buffer

[ngoldbaum]

This is a followup for #41, since I had to make a number of extensive changes relative to what that PR originally proposed, I issued a new PR instead to hopefully make this clearer.

Instead of using flexible-length arrays, the ss struct now looks like this:

typedef struct ss {
    size_t len;
    char *buf;
} ss;

Rather than store a pointer to an ss struct in the array buffer, this refactors things so we instead store the contents of the ss struct in the array buffer. This reduces the amount of pointer indirection we need to do to get access to the ss struct data and also makes it much easier to deal with empty strings, since with this new definition ((ss*)NULL)->len is zero by construction.

Instead of mallocing storage for the entire struct on the heap, we now only malloc space for the string buffer, and the caller of functions that allocate memory are responsible for handling memory for the ss struct.

To handle this change, I had to rewrite the static_string API to accept pointers to ss instances instead of returning them. I also added a new load_string helper that returns either a read-only view onto an ss struct, or if the struct is NULL, returns a view onto a statically allocated empty struct.

[seberg]

Didn't have a thorough look through, but I like this, thanks.

[peytondmurray]

🚀 This is super valuable documentation. Thank you!

[peytondmurray]

Reducing the number of stars everywhere makes this so much more readable, thanks for this! ⭐