Checking validity of names

[ggabriel96]

• added is_alphabetic, which checks that a character is within a-z or A-Z
• added is_numeric, which checks that a character is within 0-9
• added is_valid_name, which checks if a string:
  • is non-empty
  • first character is_alphabetic
  • last character is_alphabetic, or is_numeric if its length is > 1
  • all characters are alphabetic or numeric or - or _
• added is_valid_abbrev, which checks if a string is empty or is one character and it is_alphabetic or is_numeric

With that, added checks for validity of names of arguments and programs with is_valid_name and argument abbreviations with is_valid_abbrev. The check for empty argument name is separate to have a clearer message. Didn't think the same was necessary for programs.

Something "unrelated" that I've done is centralize the validations on Arg in the validate_arg function, so the factory functions construct an Arg, call validate_arg with it, and then return it.

Also added some unit tests to check that is_valid_name and is_valid_abbrev are somewhat correct. This is sorta provisional, since these functions require automated generation of input.