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.
Move lots of non-Serval-specific string functions from "serval.h"/dataformats.c
to "str.h"/str.c.
Add str_fromprint() function that reverses the effect of toprint(): interprets
escape sequences in the source string "\n\t\r\0\xHH\\" and replaces them with a
single char in the destination string.
