Move alloca_tohex() from serval.h into str.h so it can be used stand-alone.
Rename str_to_ll_scaled() to str_to_int64_scaled(). Add str_to_uint64_scaled()
and scale_factor().
Add a few more URI parsing functions. Move some functions out of str.c and
into str.h as inline functions.
Add strn_startswith() and strncase_startswith().
Make all str*_startswith() functions take const char * arguments, to make it
possible to do safe programming with consts.
