An implementation of dynamic strings based on genvector

[cher-nov]

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