Allow option-argument adjacent to option-string

[askoldilvento]

Implicit values make all these things much more complicated, but I hope I managed to not break anything and add the feature that is quite common for many Linux tools and e.g. proposed here

https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html

utility_name[-a][-b][-c option_argument]
    [-d|-e][-f[option_argument]][operand...]

With the pull request calls with adjacent options should be possible, e.g.:

arg_parse -j12  # ~ arg_parse -j 12 ~ arg_parse -j=12
arg_parse -dkj12  # ~ arg_parse -d -k -j 12
arg_parse -dkv12 34 78   # ~ arg_parse -d -k -v 12 34 78