An implementation of dynamic strings based on genvector#3

Ideas: 1. Should be null-terminated (i.e. `count` is always at least 1, and `length` is `count-1`). 2. Should have functions similar to standard ones from `string.h`, but with taking length into account. 3. Full name is "dynstring" and shortened is "dstr". 4. `typedef gvec_dstr_t dstr_t` 5. ~~Consider using another repository for this.~~ 6. Use `GVEC_MODULAR_APPROACH`. 7. Preserve compatibility with C89. 8. Alias gvec functions with `dstr_*` prefix using defines (nb: this will not break function pointer obtainment). tl;dr: steal everything from [sds](https://github.com/antirez/sds)