Reduce malloc() and free() calls about haywire HOT 2 OPEN

Unifying the mallocs in create_string:

hw_string* create_string(const char* value)
{
    int length = strlen(value);
    hw_string* str = malloc(sizeof(hw_string) + length);
    if (!str) return 0;
    str->value = (char*)str + sizeof(hw_string);
    memcpy(str->value, value, length);
    str->length = length;
    return str;
}

But to avoid uneeded copying the hw_string could have a free() function pointer so the string can be released after the use. If the string is static then it can be set with HWSTR_STATIC.

Check how this is done with SQLite for the sqlite3_bind_text:
https://www.sqlite.org/c3ref/bind_blob.html
http://stackoverflow.com/questions/1229102/when-to-use-sqlite-transient-vs-sqlite-static

The SQLITE_STATIC and SQLITE_TRANSIENT values are store in the freefn pointer (inside the string struct) and as they have values of 0 and 1 they can be identified as not being a free() function pointer. If the freefn pointer has a valid pointer, then it is called to free the string.

from haywire.

The http_request struct could have the url and body hw_string already allocated inside the struct instead of using pointers to hw_string. So we decrease from 3 callocs to 1 in create_http_request, and reduce the free() calls in free_http_request.

It could also use pre-allocated hw_strings for the headers (MAX_HEADERS).

from haywire.