- add `==` as undocumented alias of `=`
- handle negated comparison of `=` as literal
- negation generally applies to only the first expression of a Boolean chain,
except when combining evaluation of two literal strings
- Replace the parser with a recursive descent implementation that handles
parentheses and produces a stack of operations in postfix order.
Parsing now operates directly on OsStrings passed by the uucore framework.
- Replace the dispatch mechanism with a stack machine operating on the
symbol stack produced by the parser.
- Add tests for parenthesized expressions.
- Begin testing character encoding handling.
