imported kevin's three token-parser choices into core.clj
chose for comprehension with :let and :when bindings option for parse-token
Merge branch 'kevinscodereview'
literalized the declaration of the set of registers
regularized the declaration of the operators so it's a set of strings like everything else
inlined the function adding pos metadata to lexed tokens
changed str-int to use try/catch instead of a regular expression check
fixed bug in regex in valid-word function
refactored return-err function to use 'constantly'
pulled the disambiguation of minus signs out of the parsing step, put it afterwards
changed parse-token to loop and recur
removed unused 'merge-tokens' function (logic is now in disambiguate-minus-signs
got working the compiling into command-arg pairs